DFM Flex - FlexRigid

DFM Flex - FlexRigid

Wire harnesses often can be replaced by flex circuits, but in some applications replacement is just not practical.

- Simplified Flex Why consider Flex? Flex constructions Basic how to s on Flex Design Materials Terms Design related issues

- Why Consider Flex Smaller, smaller, smaller smaller A reduction in space within the design. This affects both size and weight of the overall assembly Replacing hard wiring, cables & connectors, will increase the reliability and will give a robust design Your construction require a dynamic flex cycle Flex can be custom designed to achieve impedance requirements. You can improve heat dissipation and allow more air flow inside the box. Folding is required. Folding, forming and component assembly can take place all in one platform.

- Flex Constructions Application Types - Static: Not meant to flex ( Bend & Stay ) - Flex to Install: Only meant to flex on limited basis. To install or replace and repair. - Dynamic: Depending on flex construction and conditions this could result in >1M flex cycles. Application Attributes - Controlled Impedance, by altering material thicknesses you can achieve impedances typically in the 50 100 ohm range, with a tolerance of 5 10%. Depending on construction you can affect flexibility and limit the dynamic characteristics.

- Flex Constructions.cont d Application Attributes cont d - Single Sided Flex: This is a single copper layer/polyimide clad construction with a polyimide/adhesive coverlayer laminated to one side to allow access on one side only. - Dual Access Flex: This is the same construction as single sided flex but openings are created in the base polyimide/adhesive layers to allow access to the copper on both sides. - Double Sided Flex: This is a dual copper clad consisting of two layers of copper separated by adhesive and polyimide. Plated through holes (vias) complete the continuity from layer to layer. - Multilayer Flex: This is a combination of single sided and double sided clads to achieve the total layer count desired. Vias are incorporated to connect all layers. - Rigid Flex: This can be single or multiple layers of flex sandwiched between standard PCB materials to create rigid and flexible portions

- Flex Constructions.cont d

- Flex Constructions.cont d

- Flex Constructions.cont d Please note that coverfilm on flex areas shall stopp 1-2mm inside the rigid area. This is due to risk for delamination, See next slide.

- Flex Constructions.cont d It s important to pay attention to the area where the transition is made from flex to multilayer rigid construction. Rather than have coverfilm material extend completely through the entire circuit, it is reccomended to use what s called a bikini coverfilm. Just as a bikini swimsuit only covers the essentials, the bikini coverfilm extends just far enough into the multilayer area to provide reliable adhesion. The prepreg sheet is pre-cut to nest with the coverfilm. Why is this bikini cover-film/prepreg construction important? Combined with an adhesiveless base flexmaterial, this construction makes the plated holes more reliable, because there is no adhesive in the multilayer stackup. The vias are more likely to survive temperature cycling. More important, the designer must make sure that no vias are placed in this area. As shown, the plated holes should be far enough away from the transition area so that they don t travel through the coverfilm. The distance that the coverfilm extends into the multilayer area varies with different manu-facturers, but a good rule of thumb is to place via pads no closer than within 1-2mm from the edge of the multilayer area The fig. shows how the coverfilm layer barely encroaches into the multilayer section, being replaced byprepreg in the rigid area.

- How to s on Flex Design Design tips: For first time designers and engineers it is a good idea to become familiar with the specifications that have reference to Flex Circuit manufacturing. - IPC (www.ipc.org) - IPC-2223 and IPC6013 - How do you want your flex to act or perform? - Develop your schematic. - Layer counts and trace densities will be calculated - Identify power, signal and ground traces - Understand your current requirements - Calculate conductor width needs

- How to s on Flex Design.cont d Flex Circuit Related Specifications: - IPC-2223 Sectional Design Standard for Flexible Printed Boards This specification has the most comprehensive tips for creating your design, and the following is based on this specification. - IPC-600 Manufacturability and Acceptability of Printed Circuits - IPC-FC-231 Flexible Bare Dielectrics for use in Flexible Printed Wiring - IPC-FC-232 Adhesive coated dielectric films for use in cover sheets for Flexible Printed Wiring - IPC-FC-233 Flexible Adhesive Bonding Films - IPC-FC-241 Metal clad flexible dielectric for use in fabrication of Flexible Printed Wiring - IPC-4562 Metal Foil for Printed Wiring Applications - IPC-T50-D Terms and definitions

- How to s on Flex Design.cont d Material Selection: - Attributes to consider flexibility, impedance, emi, to name a few. - Thinner material does not equate to cheaper. In most cases the thinner the copper and or polyimide / adhesive, the more expensive it is. BASE MATERIAL - The base material is the flexible polymer film that provides the foundation for the laminate. Under normal circumstances, the flex board base material provides most primary physical and electrical properties of the flexible board. - In the case of adhesiveless board constructions, the base material provides all of the characteristic properties. Although a wide range of thickness is possible, most flexible films are provided in a narrow range of relatively thin dimension, from 12 µm to 125µm. - While it is recognizable from experience that thinner materials are more flexible, it is worth remembering the engineering principle that with most materials, stiffness is proportional to the cube of thickness. This means that if the thickness is doubled, the material becomes eight times stiffer and will only deflect one-eighth as much under the same load.

- How to s on Flex Design.cont d Component termination method: - Surface mount, through hole, type of external connection, component support etc Level of electrical testing needed to assure that your design is acceptable: - Open and short test - Full functional test of all components.

- How to s on Flex Design.cont d -Your ultimate design is a collaboration across multiple engineering disciplines, Electrical, Mechanical and CAD. - Paper Doll -This term is used in conjunction with mechanical mock ups. This is a cost effective way of demonstrating form and fit and in most cases a good safety net to catch potential mechanical issues. - Trace/Space -While this is one of the building block of your design, special care must be given to the actual trace structure. Care must be taken to avoid sharp corners that is more typical on a rigid design.

- How to s on Flex Design.cont d -Filleted pad/ Hold Down Tabs -A major location for failure in a flex circuit is the interface between the board trace and the pad at the end. It is extremely critical that these interface points have transitional areas more commonly referred to as fillets. Hold down tabs add robustness to the copper pad. The additional hold down tab keeps the pad anchored during any subsequent assembly and rework. - Solder Relief -When placing pads in large copper traces or ground planes, solder relief pads should be created to allow for easier soldering of components. Large copper areas will sink heat away and cause cold or weak solder joints.

- How to s on Flex Design.cont d - Tear Stops - All inside corners MUST have a radius. In these cases a tear stop should be incorporated into the design as an added safety feature to guard against tearing or cracks propagating across a flex circuit. - Shields and Planes - Cross hatching can be used to create a shield or plane when flexibility is still a requirement. Shields can be created using copper or they can be screened on using conductive silver epoxy. - Grain Direction - When designing dynamic flex the copper grain direction must be taken into account. Rolled Annealed copper has a specific grain direction. To maximize the copper life you must flex in the direction of the grain.

- How to s on Flex Design.cont d - Material Utilization - This is the number one cost contributor to your design. During production, most manufacturers use a panel format. The more up per panel the lower the cost. Your goal should be the smallest form factor as possible. This can be achieved several ways but one creative way is to incorporate folds into your design. - Consider how flex-boards will be nested on a panel. - Follow recommended tolerances whenever possible. - If flex-boards have only a few layers, using stiffeners can be far less expensive than a rigid-flex board. - Remember Flex-boards both require and permit looser tolerances than rigid boards.

- How to s on Flex Design.cont d I-Beam - When routing your traces in a double sided flex it is best to stagger the traces to prevent an I-Beam effect. With the conductors stacked you run the risk of compression issues when bending to install. - Balance the track weights and material thicknesses on each side of the neutral bend axis. - Tracks should always be routed through bend areas as close to perpendicular (90 deg) as possible. - Track thickness and width should remain constant in bend areas. - Plated through holes should be kept out of the bend areas.

- How to s on Flex Design.cont d Conductor aspect - In a perfect world that rule of thumb is 4 times the track thickness (T). This is a safe rule, however your design will be pushed to ever increasingly smaller trace and space geometries. Ideal: Track width greater than 4x track thickness.

- How to s on Flex Design.cont d -Balanced Board - A balanced board is important when flexibility is a concern. The copper conductors must (if possible) be placed in the neutral axis of the flex cross section to provide optimal performance. This allows the flex to operate without compression or tension while flexing

- How to s on Flex Design.cont d Electrical Considerations - Predictable electrical characteristics make flex boards an ideal choice for high-speed signal transmission. - Uniform spacing between tracks and grounds, continuous shield layers, and repeatable geometries are features that help control impedance and reduce crosstalk. - With flex boards, you can eliminate connectors and other transitions that contribute to signal attenuation. - Manufacturers can provide tight tolerances on track width, spacing, and distance to ground layers in order to meet your impedance requirements. - Actual impedance will also depend on the board s shape after installation. - Contact your manufacturer for advice on designing boards to specific electrical characteristics.

2-Layer vs. 3-Layer Designs2-Designs 2-layer Designs Provide More Flexibility - Require careful pad breakout design to maintain good return path integrity. - Lowest cost - Most non-emi critical applications can benefit from 2-layer flex designs 3-layer Designs Provide Better Shielding - Require cross-hatch to maintain flexibility - Require careful trace routing and pad breakout design to maintain good impedance and crosstalk control. - Highest performance

Multi-layer Requires Cross-Hatching of Planes to Maintain Flexibility - This can reduce performance if not well-designed. Two-layer can Use Solid Plane - Has performance advantage over 3 -layer - Wider traces for microstrip - Signals travel parially in air, wich has low loss. - Has increased emissions - Can be controlled by good design and layout practices

Traces misalignet with Cross Hatch

Microstrip sensitivity (4.5mil 0.11mm)

Copper vs. Silver inc. shilding

Silver Ink Flex Design Lower cost May be used for economic shielding in EMI sensitive applications - May be used over two or three layer copper flex for additional shielding in Mil/Aero applications Single-Ended Applications - Use below 250 MHz Differential Applications - Use below 1 GHz (2 Gbps) Current carrying power and ground should be placed on copper layers Limited flexure cycles

Insertion Loss ComparisonInsertion Comparison

- How to s on Flex Design.cont d Current Calculator - There are several ways to calculate current carrying capacity. One of the most popular is to use the nomograph published in IPC-2221. There is a version that can be accessed at: http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/

- How to s on Flex Design.cont d Bend Radius: Single-layer: Double-layer: Multilayer: 06 board thickness (minimum) 12 board thickness (minimum) 20 board thickness (minimum) Circuit thickness is approximately 0.15mm per layer. Sharper, permanent bends are common for bend and stay applications Strain vs. Ductility: The bending of flex boards causes deformation, strain, and stresses in the circuit materials. The strains and stresses are functionally related for each material via the stress-strain diagram. The forces necessary to cause flexural deformation are small; the flexural deformation is determined by the imposed bend geometry.

- How to s on Flex Design.cont d Basic Manufacturing Flow-The sample flow depicts the process steps required to build a double sided- and Rigid-Flex circuit. Several inspection steps, as well as sub steps have been left off.

- Materials Material Selection Materials can range in a variety of attributes. You must have a clear understanding of exactly what type of performance you expect and/or desire in your application. Is a high flex life required, controlled impedance, ultra thin coppers, adhesiveless clads, or is it solely a cost driven design? The industry leader in flex circuit materials, DuPont Electronics. (www.pyralux.com) is often used.

- Materials.cont d Polyimide Films. - This is the material used as the base layer for both the copper clad and the coverfilm. The most common thicknesses are: ( Red is less common ) 0.0127mm - 0.025mm - 0.050mm - 0.075mm - 0.127mm Adhesives - Adhesives are used to bond the polyimide film to the copper layer or board layer it is exposed to. Some of the typical adhesive types are: Acrylic, Modified Acrylic, Phenolic Butyral, Modified epoxy and PSA (Pressure sensitive adhesives). The most common thicknesses are. 0.0127mm - 0.025mm - 0.050mm - When it comes to temperature resistance, the adhesive is typically the performance-limiting element of a laminate, especially when polyimide is the base material. - As with base films, adhesives come in different thicknesses. The thickness selection is typically a function of the application. For example: Different adhesive thicknesses are commonly used in the creation of coverlayers in order to meet the fill demands of different copper-foil thicknesses that may be encountered. It is essential that you verify the chosen material availability from your manufacturer.

- Materials.cont d Metal (copper) Foils - A metal foil is most commonly used as the conductive element of a flexible laminate. The metal foil is the material from which the circuit paths normally are etched. Typical flex boards use RA (Rolled Annealed copper), although ED (Electro Deposited) coppers are used in adhesiveless clads. The thickness range can vary, but typical RA foils are 0.017mm (½ ounce), 0.035mm (1 ounce) and 0.070mm (2 ounce). LAMINATES - Metal foil clad laminates are the basic form of material used for most flexible board constructions. While copper foil dominates applications, other foils can be used. Typically, a laminate is created by bonding together a sandwich comprised of a base material, an adhesive and a metal foil. The stack is then subjected to sufficient heat and pressure in a laminating press to create a permanently bonded metal polymer laminate. In the case of adhesiveless laminate substrates, the adhesive is absent from the construction. Adhesiveless laminates can withstand higher temperatures.

- Materials.cont d LAMINATED COVERLAYERS - Normally, a coverlayer is a two-layer material comprised of a base material and a suitable thermosetting adhesive. However, suitable homogenous thermoplastic films may also be used as coverlayers. The coverlayer serves to protect the conductors of the finished flex board and help enhance flexibility. PHOTOIMAGEABLE COVERLAYERS - Another type of coverlayer for use in flex manufacture is a photoimageable coverlay. This product is akin to dry film solder mask. It requires vacuum lamination to assure a good seal around the board traces. Just like a photoimageable solder mask, the material is exposed and developed to provide access to board component attachment features..

- Materials.cont d COVERCOATS - The term covercoat is used to describe a range of thin coatings applied to the surface of conductors in lieu of a coverlayer. Although some suppliers of covercoat material have some impressive flex cycling data, covercoats are normally reserved for applications where no (or minimal) dynamic flexure is required. In manufacturing, covercoats normally are applied as a liquid by screen-printing and then either cured by heat or by exposure to UV radiation.

- Materials.cont d BONDPLIES - Bondplies are flex board construction elements comprised of a base film with an adhesive film cast on both sides. The adhesive element of the construction is typically a thermosetting material. While bondplies can be used for creating two-metal layer laminates, bondplies most typically are used as building blocks in the manufacture of some more complex flex structures such as multilayer flex and rigid flex boards.

- Materials.cont d CAST ADHESIVE FILMS - Cast adhesive films are freestanding adhesive materials. The films are most typically thermosetting adhesive films, which are cast onto a disposable carrier or release film. Cast adhesive fims are also often used to bond rigid stiffening materials to a flex board. They have also been used as replacement for bondplies in certain applications. PRESSURE-SENSITIVE ADHESIVES - Pressure-sensitive adhesives (PSAs) are a family of semi-permanent to permanent adhesive films on peelable carriers, which can be transferred directly to the surface of the flex board or other material for later attachment to another surface. Once the PSA is applied to the board, it can be bonded later to nearly any surface. However, most typically, pressure-sensitive adhesives, when used in flex circuit manufacture, are employed to attach stiffeners to flexible boards. There is also a special subset of the pressure-sensitive adhesive family that can be screen-printed directly onto the back surface of the flex board, and then cured to provide the necessary tacky finish by exposure to ultraviolet radiation. This method can be used to advantage in very-high-volume applications or where cost is kept to a desired minimum.

- Materials.cont d STIFFENER MATERIALS - While not an integral part of the flex board, stiffeners are an important element of flex board construction. Stiffening materials are used to reinforce flex circuits when and where required. Most commonly, stiffeners are under areas where electronic components are to be attached. They serve to support the weight of the components both through the assembly process and in the application. Stiffeners can be made of almost any material, including metal, plastic, resin-glass laminates or even additional layers of coverlayer material. The use of coverlayer material to stiffen areas of a flex is actually a very common practice. COMPONENT HOLES IN STIFFENER - Holes in the stiffener for through hole mounting of components, should be 250-375µm larger than the through holes in the flex board.

- Materials.cont d FILLET TRANSITION EDGE OF STIFFENER - Filleting of the transition edge of a stiffener with a resilient adhesive or epoxy is another common method of strain relieving boards. The small bead of a suitable polymer will provide a simple means of transitioning strain from the stiffener to the flex board.

- General Flex Circuit terms -Basic Glossary of Terms (see IPC-T50-D for complete list) Adhesive -Acrylic*, flame retardant, epoxy, epoxy prepreg, polyimide prepreg, phenolic ( * most common materials used for manufacturing flex circuitsfor max flexibility.) Annular Ring - This is the exposed copper ring surrounding a through hole. Artwork - This is the tool used to define circuit features. Typically CAD generated. Back Bared Pads - This is a flex that is typically single sided in construction but has the copper accessed from both sides. Back Side Access - This is a flex that is typically single sided in construction but has the copper accessed from both sides. Conductor / Track - The metallic path that carries electrical current.

- General Flex Circuit terms Cont d Track / Conductor Spacing - This is the distance between tracks measured at the foot (widest point) of the track. Track Conductor Width - This is the width of the track measured across the base of the track. Coverfilm - This is the insulating material, usually in a film format, that is laminated to the flex board to define solderable areas. Can be drilled, laser cut or chemically removed to create complex geometries. Dynamic - This is the term used to describe the intended use of the flex. Repeated flexing, as in a disk drive, requires a dynamic flex circuit. ED, Electro Deposited -This is the means by which copper is deposited on polyimide to create an adhesiveless clad.

- General Flex Circuit terms Cont d Hard Tooling - A term used to describe tooling used to fabricate flex boards typically when tight tolerances and or high volumes are required. HASL -This is the acronym for a soldering process. Hot Air Solder Level. (Often not ideal to use on flex boards) Hold Down Tabs -These are extensions of the copper pad and are used to help anchor the pad during soldering processes both in assembly and rework. I-Beam Effect -This is the result of stacking your conductors during your design. Subsequent flexing induces stress and compression, causing cracking to occur. Impedance - Basically this is the total effective resistance to AC current. It can be altered by changing conductor spacing, signal spacing relative to ground, material thicknesses.

- General Flex Circuit terms Cont d Nested Panel - This is the process by which the individual board is maximized on a panel. Pad - The end of a conductor usually configured to accept a component or to be attached to a device. PIC, Photoimagable Coverfilm - This is another means of defining your solderable areas. This material is apply in a clean room environment, exposed with an artwork and then developed. This allows for complex geometries not available from routing or other soft tooling means.

- General Flex Circuit terms Cont d Pads only Plate (button plate) -This is the process by which only the via and a small portion of the surrounding pad is plated in a double sided or multilayer flex. This limits the amount of plating a panel will see and allows the double sided flex board to still be somewhat flexible.

- General Flex Circuit terms Cont d Panel Plate - This is where the entire panel is plated increasing the total overall copper thickness.

- General Flex Circuit terms Cont d PSA (Pressure Sensitive Adhesive) - An adhesive used to bond stiffeners to specific regions of the flex circuit. RA (Rolled Annealed) - This is a term used to describe the type of copper and it is actually the method by which the copper is flattened. Through Hole/Vias - A plated hole in the panel that can be accessed through the coverfilm or can be tented over with the coverfilm. There are numerous versions of these. Blind Via - Typically a via that penetrates to several layers below but is covered up by the coverfilm Buried Via - A via that is set deep inside a multilayer with no access to the coverfilm Tented Via - A via that goes completely though the flex, but is covered by the coverfilm on both end limiting access.

- Capability Some safe values to use for design Panel size (is manufacturer dependent - Single sided through multilayer construction, 12 x 18 Drilled holes size - Plated or non-plated 0.25mm diameter - Tolerance of hole to hole is ± 0.1mm Aspect ratio (ratio of hole depth/hole diameter) - 10:1 maximum. Track width /space - Using ½ ounce (17um) thick copper, 0.1mm track/space - Using 1 ounce (35um) thick copper, 0.15mm track/space Conductor aspect ratio - For best producibility, design traces to be at least four times as wide as they are thick. In tight situations it can be successful to achieve 2.5-3 : 1 ratio conductor widths. Layer count - 1 10 layers - Solid or cross hatched ground layers

- Capability Some safe values to use for design Stiffener - Epoxy-glass (FR-4), polyimide-glass, polyimide, copper, aluminum. Surface finishes - ENIG - Chemical Tin - Chemical Silcer - Hard Gold over Nickel - HASL / LF-HASL **** - OSP Outline tolerances: - SRD 0.38mm - Laser / Hard tool 0.10mm - CMD (chemical Milled die) 0.25mm Shield layers - Solid or grid patterned - copper foil or screened conductive ink.

- Capability Some safe values to use for design Fingers - Supported: 0.20mm minimum, center-to-center; - Unsupported: 0.50mm minimum, center-to-center.

- Design issues TEAR-RESISTANCE FEATURES IN FLEX DESIGN - All flex board designs should be made as tear-resistant as possible. While the material may not be intrinsically tear-resistant, tear-resistance can be improved by employing certain features in the design. There are several possible methods, described and illustrated below. All of the following techniques have been successfully used to help prevent tearing. One or more of the following techniques can be used: 1) Radius All Internal Corners The first line of defense against tearing is to make certain that all internal corners are provided with as generous a radius as possible. This design practice is the most important and simplest of all methods used to prevent tearing of the flex buard material.

- Design issues.cont d 2) Laminate Glass Fabric in Corners Glass cloths can be laminated into corners during the fabrication process. Though not flexible, this method has been shown to provide a very robust corner construction and has been favored in the past by military product designers. It is an expensive solution, however, because of the type of preparation required and should be used only after careful consideration of the alternatives. Use Fluoropolymer Coverlayer The use of fluoroplastics such as Teflon as coverlayers helps to improve tear resistance by virtue of the high tear resistance of the polymer itself. This is due to the fact that fluoropolymer tends to stretch rather than tear, adding toughness to the substrate. An additional benefit of using fluoropolymer coverlayers for those involved in high frequency design is that the dielectric constant of the coverlayer is much lower.

- Design issues.cont d 3) Use of Radiused Slots The use of slots with ends that have a radius to access relieved board features also can serve to provide tear resistance. Normally, such features can easily be provided for during the punching operation or other board fabrication process. 4,5) Drilled Holes at Corners or Ends of Slits Drilled or punched holes in corners or at the ends of access slits have been used with success when flexible appendages must be spaced close together. This method allows the greatest use of material, but the hole size chosen will impact tear resistance. If the hole is very small, the overall robustness will be reduced.

- Design issues.cont d 6) Aramid Fibers Inside Cut Line As an alternative to glass cloth, the use of aramid fibers routed through corners or along the entire outline of the flex board is a unique method to stop tearing. The thin polymer fibers have very high strength and are very pliable, minimally affecting flexibility. However, this is a labor intensive method and should only be specified with the knowledge of cost impact. 7) Leave Metal in Corners The board design should, if possible, have small areas of copper provided for at internal corners to serve as tear stops at the inside of corner radius. This serves to prevent further or imminent propagation of a tear through the polymer, should a tear in the material start.

- Design issues.cont d (1)Large radius in corner (2)Embedded glass cloth (3)Radiused slot (4)Hole in slit (5)Drilled hole at corner (6)Embedded aramid fiber (7)Extra copper in corner

- Design issues.cont d Proper circuit nesting can greatly improve panel yield and lower overall cost. If folding can be tolerated as an assembly operation, yield can be maximized. For dynamic flex board designs, the grain direction requirement may impact layout.

- Design issues.cont d Conductor width nomograph The nomograph is valid only for tracks with a polyimide cover layer not exposed conductors. The temperature rise curves only recognize heat generatedby the track itself. Heat from power dissipating components or nearby tracks on other layers is not included. Example #1: A current of 1 amp with ½ oz. copper and 30 C temp. rise, will require a track width of 0.040 (1mm). Example #2: A 0.140" (0.36mm) wide track etched from 1 oz. Copper (0.0014") will produce a temperature rise of 10 C at 2.7 amp.

- Design issues.cont d It is best to specify round (instead of slotted), through-holes. This will reduce drillingtime and cost. Whenever possible, design pads larger than the access holes. If space is critial, use hold-down tabs. Hold-down tabs are especially important for single layer boards, because a single layer board does not have the added strength of plated through-holes. A variety of hold-down tab designs are available. Pad fillets improve etched yield and material strength. Fillets are appropriate when the pad diameter is greater than the connecting strand width. Acute angles at the interface between traces and pads are to be avoided by using fillets to minimize the concentration of stress at the interface.

- Design issues.cont d The steel rule die is a cost-effective tool for punching flex boards from their panel. Class-A tooling for flex circuit punching offers both high precision and long tool life. Example of an etched punch die tool

- Design issues.cont d Stiffeners in panelized form facilitates both the application of the flex board to the stiffener and subsequent component assembly. When the flex board and stiffener are bonded together, the resulting flex board panel can be processed much like a rigid board. Note that the adhesive is applied oversized and cut to dimension before the routing step is performed.

- Design issues.cont d STAGGERED LENGTH BOARDS (BOOKBINDER CONSTRUCTION) For ease of flexing multilayer and rigid flex designs, the use of staggered length design is commonly employed. The technique is accomplished by adding slightly to the length of each succeeding flex layer, moving away from the bend radius. Staggered length designs facilitate bending of the flex board. The circuit can only be bent in one direction by design. A common rule of thumb is to add length equal to roughly 1.5 times the individual layer thickness. Without staggered lengths, layer buckling occurs.

- Design issues.cont d Various flexing and bending methods large radius or hinge type flex Coiled flex Counter Rolling flex window shade flex Accordion flex Rolling flex

- Cost impact of layer count The information for the chart are taken from a sample of circuits built with standard materials. This chart is not intended to be used as a price guide. However, it does showthat circuit cost generally rises with layer count. It is in your best interest to consider all options to minimize cost. For example, use two circuits to do the job of one. Two double-layer circuits may be less expensive than one four-layer circuit. But the cost savings of the circuit may be offset by additional assembly requirements. Circuits can also be folded in order to save space and layers. Each situation is unique. A rela-tively small amount of time invested in project assessment canresult in significant savings overall.

- Documentation Needs for Flex Boards The documentation package for a flexible board is the means by which the needs and requirements of the user are communicated to the manufacturer. If the information presented is clear and easily understandable, the chances of getting the product you want on the first pass is greatly enhanced. But we have an idea that 80% of the flex documentation packages we receive require some sort of clarification. This is directly man-hours lost that could have been used to meet manufacturing targets. At a bare minimum, the circuit manufacturer needs to be given the following information in order to complete his manufacturing task properly:

- Documentation Needs.cont d -Class of product -Materials to be used for construction - base material and coverlayers - metal foil type and thickness - Definition of holes, hole position and hole size - Stackup showing no. of layers & cross sectional view - Marking requirements, materials and locations - Bend and Flex locations and direction of bend - Stiffener locations and bonding requirements - Special process and/ or finish requirements - Test point locations - Special testing requirements - Coverlayer or Covercoat opening locations - Board outline with datum and dimmensions - Tolerances for manufacturing - Hole tolerances - Physical dim. Tolerances - Critical dimensions, if any. The list of items just provided provides a solid base in terms of understanding the general nature of need for flex board design documentation. However the list does not detail all the potential documentation requirements or needs of all possible flexible circuit designs. This is a never-ending list. Designers will always have special needs for their particular board design.

- Documentation Needs.cont d DEFINE CLASS OF PRODUCT There are three generally-accepted classes of product as defined by IPC standards: Class 1, consumer products, Class 2, telecommunications, computers and general industrial, and Class 3, high-reliability. The definition of class will serve to provide guidelines as to how the product must be fabricated and inspected as well as provide performance requirements. DEFINE MATERIALS TO BE USED FOR CONSTRUCTION The materials to be used in construction of the circuit need to be defined to inform the manufacturer of which material the circuit should be made from. This includes issues of base polymer choice, adhesive type and copper foil type, along with the thickness of each of the above.

- Documentation Needs.cont d Definition of holes, hole position and hole size There is a need to define a hole count for each different hole size called out. This can usually be most easily accomplished by simply extracting data from digital files. The data are commonly used to help define the manufactured cost of the board. While the sales department may be interested in the hole count, manufacturing is interested in hole location in order to verify that holes and artwork are properly aligned. Hardcopy or digital information will work. These data can be extracted from drill files. Stackup showing no. of layers & cross sectional view This information serves the needs of both manufacturing and sales. The layer count provides an indication of board complexity to sales and a key descriptor of the product to manufacturing. The cross-sectional view of the board is required to provide a visual cue as to what the designer expects his finished product to look like on edge. This is helpful predicting overall thickness.

- Documentation Needs.cont d PROVIDE COVERLAYER OR COVERCOAT OPENING LOCATIONS The documentation package should also define the location of access points through the coverlayer or covercoat. In many cases, these will match the hole locations defined earlier, however, when SMDs are used, many other locations on the circuit will require access. PROVIDE CIRCUIT OUTLINE WITH DIMENSIONS AND DATUMS The final circuit outline is necessary to define the periphery of the circuit relative to the circuitry itself. These data are used to create the tooling required to remove the part from the panel. The datums are important reference points to facilitate measurement, and datums shall be inside the board rather than outside the board outline. This makes inspection easier.

- Documentation Needs.cont d MARKING REQUIREMENTS, MATERIALS AND LOCATIONS Marking requirements must be defined to provide the circuit manufacturer the information needed to locate accurately and mark properly specific locations on the circuit. The choice of type and color of marking ink must also be defined. BEND, FLEX AND CREASE LOCATIONS It is helpful to define the location of bend and flex areas as well as where crease lines may be required. This can be accomplished by placing special indicating features in the circuit artwork. It can also help the assembly process by providing information as to which direction a bend must take. For example, dotted lines could be used to indicate bends in one direction, and solid lines could then indicate bends in the opposite direction.

- Documentation Needs.cont d STIFFENER LOCATION AND BONDING REQUIREMENTS Location of flex circuit stiffeners and special bonding requirements or instructions should be provided in the documentation package. Requirements for special strain-relief techniques such as an epoxy bead along flex to rigid trans formations should also be mentioned here. SPECIAL PROCESSING AND/OR FINISH REQUIREMENTS If special processes are required such as the addition of special tear restraints or special finishes on the flex board, they should be marked. This includes request for solder plating, nickel or gold plating, and the use of special, Organic Solderable Protective coatings. The chemical surfaces is preferred on flex boards, due that molten solder finishes (HASL) can damage the flex board.

- Documentation Needs.cont d TOLERANCES FOR MANUFACTURING Manufacturing tolerances should be called out on the print. Most print formats provide a tolerance block near the title block for the drawing. Keep in mind that by nature of the product, flex circuit tolerances are or should be less stringent than those applied to rigid board constructions. As a result, the tolerance block should accurately reflect the capabilities of the finished flex product. TEST POINT LOCATIONS Test point locations, if restricted, should be defined either in the print or, more preferably, in a digital data format. The use of test nodes in place of 100% testing of all points on a circuit can help reduce testing cost by limiting test points only to those required. It is important to remember that test probes will likely leave physical indentations on the metal surface due to the softness of the flex circuit base material. If this will be cause for concern by receiving inspection, it should be fully discussed with the manufacturer. SPECIAL ELECTRICAL TESTING REQUIREMENTS Should they be required, any special electrical tests should be defined in the documentation. This includes such tests as TDR (Test Discrepancy Reports) or special high pot testing. Conditions of test, and test point locations, should also be thoroughly defined in advance.