FLEXIBLE CIRCUITS MANUFACTURING

Transcription

1 IPC-DVD-37 FLEXIBLE CIRCUITS MANUFACTURING Below is a copy of the narration for DVD-37. The contents of this script were developed by a review group of industry experts and were based on the best available knowledge at the time of development. The narration may be helpful for translation and technical reference. Copyright, IPC, Inc. Association Connecting Electronics Industries. All Rights Reserved. Cellular phones, lap top computers, cameras -- what these products have in common is they all use flexible circuits instead of rigid printed wiring boards for their interconnections. The main reason for using flexible circuits in these products is they can be bent and formed for ease of assembly. Think about a lap top computer. Imagine how a flexible circuit is threaded through the hinge connecting the processing unit to the display. Flexible circuits are light weight and can fit into very tight spaces, solving a wide range of interconnection problems. These circuits can change width several times, fold, twist and even flex into a rolled up configuration. The complexity of flexible circuits ranges from the single sided variety to the complex multilayer circuits found in today's advanced electronic equipment. This videotape will explain the typical manufacturing processes for single sided, double sided and multilayer flexible circuits. Let's begin with a single sided flexible circuit. There are two types of processing models -- sheets and rolls. We'll start with the sheet format, also called panels. The base materials are called copper clad flexible laminate. The laminate is typically made up of an unreinforced film that is coated with adhesive. There are also metalized films without adhesive. One of the outside surfaces of the laminate material is covered with a thin sheet of copper foil. This sandwich is bonded together under heat and pressure during a lamination process. Copper clad flexible laminate is usually ordered in large sheets. 24 by 36 inches is one standard size. It can also be ordered in specific panel sizes. The first step is to make sure that the laminate material is the correct size and thickness. It may also be checked for imperfections in the copper surface, such as pits and dents. Some companies prefer to rely on statistical data -- developed by the laminate manufacturer -- to ensure that all laminate material meets their procurement standards. 1

2 Flexible material handling techniques are crucial to the successful processing of the circuits. Techniques and equipment may need to be modified to support these thin laminates. After the size and quality of the laminate has been certified, the full size sheets need to be cut down to a usable panel size. This is typically done on a shearing machine. Many companies use a standard fabrication panel size - such as 18 by 24 inches -which fits optimally on all of their production machines. Before we process the laminate any further, we may want to prepare the panels by baking them in an oven. A controlled heating and slow cooling process will remove moisture and help balance any internal stresses evenly -- from panel to panel. At this point some single sided panels are drilled. Drilling will be discussed in detail during the double sided flexible circuit description. The panels are now prepared for the application of the conductive pattern or circuit image. This surface preparation can be performed chemically by dipping the panels in an acid bath, followed by an anti-tarnish agent, chemically cleaning, or micro-etch; or mechanically by scrubbing the panels to remove oxidation. This also provides a slightly roughened surface to improve the adhesion of the imaging resist. Now we're ready to create the conductive pattern on the copper side of the panel. The first step is to apply the image -- using either dry film, a liquid photoimagable resist, or a screen printing process. For our illustration we will explain the dry film method for conductor imaging. Later on we'll explain another imaging technique for solder mask application. The dry film laminator uses heated rollers to press a photoresist onto the slightly roughened surface of the copper. Now we're going to place a sheet of film that contains a negative image of the desired conductor pattern, or land pattern, onto the resist coated panel. If everything looks evenly centered, the sandwich is inserted into the exposure chamber, and a UV light source is activated. The UV light will pass through the clear areas of the phototools. Wherever the UV light hits the photoresist, a chemical reaction takes place. The photoresist will expose or harden in those specific areas. The areas where the UV light does not penetrate through the film and into the photoresist will remain relatively soft or unexposed. Next we remove this soft or unexposed resist in the developing process. This will reveal the unwanted copper in preparation for the etching process. The etching process will remove the unwanted copper from the panel's surface. This is done with a chemical solution. What remains on the panel is the copper circuitry that is underneath the photoresist. Next, this hardened photoresist is stripped away, leaving the remaining copper in 2

3 the desired circuit pattern. At this point, there is usually some type of inspection process -- either visual, with automatic optical inspection equipment, or by an electrical test process. Now we will mask off all the areas of the flexible circuit that will not be coated with solder. This is done with a coverlay or a solder mask. The coverlay is an adhesive film that is prepared by drilling holes or punching the desired solder mask pattern. There are different techniques for drilling the coverlay due to its plastic characterisitcs. The release sheet should be removed before the coverlay is laminated onto the flexible circuit. Controlling the dimensional changes in the base material is one of the most critical steps in the manufacturing of a flexible circuit. Care must be taken to select lamination parameters, release sheets and press pads that will evenly distribute heat and pressure. Tight controls on press time and temperature, heat ramp rates, and cooling are critical to consistent dimensional performance. When solder mask is used in place of coverlay, selection of the solder mask is critical for flexibility. A common technique for applying solder masks to flexible circuits is the liquid photoimagable solder mask. It can be applied in a number of different ways including screen printing, curtain coating or electrostatic spraying. Dry film solder mask may also be used for flexible circuits. Regardless of the solder mask application technique, the coated panel is exposed to a photographic image of the desired solder mask pattern. A developing process then removes the unexposed material -- leaving the solder mask image on the surface of the panel. A cure cycle is done following the developing operation. After the coverlay or solder mask has been applied, we may then coat the exposed copper with molten solder. This process is called hot air leveling. An alternative to hot air leveling is selective plating. Selective plating can be performed with metals such as nickel, gold and tin-lead. The other model for single sided flexible circuit fabrication is called roll processing, or reel-to-reel. Roll processing is usually done in high volume applications. Let's take a look at the differences. The laminate is made and used in continuous rolls. Roll widths vary from 9 to 24 inches. Notice that the rolls are not baked prior to processing and there are no panelizing steps prior to imaging. The rolls are imaged by either screen printing, or exposing and developing dry film photoresist which protects the conductors. Some tooling holes are punched before imaging. Usually optical targets are imaged and the tooling holes are formed after imaging. The final component holes can be formed by laser, or by mechanical punching. Let's stop for a moment to answer any questions you may have. 3

4 Now we'll go back and examine the differences in manufacturing double sided and multilayer flexible circuits. In double sided flexible circuits, both the outside surfaces of the laminate material are covered with thin sheets of copper foil. This means that components can be placed on both sides of the flexible circuit. It also allows much more trace circuit density and complexity. The process for a double sided flexible circuit is similar to a single sided panel. Let's review the sequence and look at the differences. First, the material is prepared. Prior to drilling, a predetermined quantity of flexible laminate is placed between phenolic fiberboard for support. Now we'll drill two or more tooling or registration holes along the edges of the panels. These holes will be used to line up all the different features on the panel, including the conductive patterns and the drilled holes. The tooling holes will also align the fabrication panels on all of the production machines during each step of the manufacturing process. Next we'll use the tooling holes to align each of the panels on a numerically controlled drilling machine. Each drill machine usually has several drilling heads with corresponding tooling pins. The computer that controls the drilling machine is programmed to move the table to the correct location in both the x and y directions. Then the drilling head will create the hole in the proper location. After all the holes of one size are drilled, the drill heads will automatically discard the used bit and pick up the next size drill. There may be hundreds or even thousands of holes in each panel... and several different hole sizes. Automatic punching may be used in place of drilling for high volume applications. In this manner, holes for the entire circuit can be punched all at once. The panel may now be inspected to make sure that all the holes are in their correct locations. Another concern during the drilling process is the formation of burrs, or small protrusions of copper, around the edges of the holes. Burrs are created when the drill bit pushes a small piece of copper away from the hole instead of cutting it off. The holes can be deburred mechanically by scrubbing the copper surface with brushes, or with some type of abrasive compound. Care should be taken during these deburring processes not to damage or distort the laminate. Also, machinery should be set up specially for thin laminates. At this point, a new processing step is introduced. A very thin film of electroless copper is chemically deposited over the entire surface of the panel -- and inside the holes. Once the hole walls are metallized like this, we have created an electrical connection from one side of the flexible circuit to the other. In place of electroless copper, there may be other techniques such as a conductive polymer coating, or direct metalization. The steps for imaging and developing are identical to the process for single sided boards. For imaging, we utilize the same tooling holes that we used earlier to drill the holes. This way everything should continue to line up properly. After developing, the hardened or exposed photoresist that covers the rest of the 4

5 panel will be used during the next step --to block or resist electroplating. During the electroplating process, we will be depositing additional copper and a thin coating of tin-lead on top of the exposed copper which will act as an etch resist. With flexible circuits, the copper's ability to withstand cracking is critical to maintain flex life. After electroplating, the remaining photoresist is removed in the stripping operation. The stripping chemistry will dissolve the hardened resist, exposing the bare copper beneath it. At this point the panel has the desired conductive pattern, overplated with tin-lead etch resist. The rest of the surface is still covered with copper. The etching process will now remove this unwanted copper. The tin-lead overplate on top of the conductors -- and inside the holes -- will protect the copper underneath it from the etching chemistry. After the etching process, the conductive pattern begins to resemble the finished product. An alternative to this type of processing is to electroplate the entire surface of the panel instead of selectively plating an imaged area. At this point, the dry film resist is laminated onto the panels, and the panels are then imaged and developed. The copper is then etched away between the circuit traces. After the panels are inspected, the tin-lead we previously electroplated must be stripped off to prepare the flexible circuit for the coverlay or solder mask operation. Coverlay or solder mask is done in a manner identical to the single sided board process. The processing to this point is completed by the hot air level operation. Let's stop once again to answer questions you may have. Now let's take a look at multilayer flexible circuits. Since several layers of circuitry are laminated within the flexible circuit, this type of processing offers more potential interconnections per unit of area for packaging electronic components than do double sided boards. Multilayer flexible circuits range in layer count from 3 to 30. The innerlayers of the multilayer flexible circuit are processed first. Again we start with a copper clad laminate. This material is cut to size in a manner similar to single or doubled sided panels. Laminated onto both sides of the copper foil is photoresist -- in this instance dry film. The photoresist will be used to create the circuit image or conductive pattern on the innerlayer. A negative photographic image of the desired conductive pattern is carefully registered onto both sides of the photoresist laminated panel. A UV light then shines through the clear or transparent areas of this phototool. The photoresist that is exposed by the UV light will harden. The areas of photoresist that are not exposed to the ultraviolet light will remain soft and ready to be dissolved away. 5

6 The developing process will now remove or strip away this soft or unexposed resist. This will reveal the unwanted copper in preparation for the etching process. The etching process will remove the unwanted copper from the innerlayer surface. This is done with a chemical solution. What remains on the panel is the copper circuitry that is underneath the photoresist. Next, this hardened photoresist will be stripped away, leaving the remaining copper in the desired circuit pattern. The panels are now inspected, usually using automatic optical inspection to make sure there are no open or short circuits. Faults, or defects that are marked during this inspection are then confirmed by the operator at a verification station. At this point there may be a coverlay or solder mask operation to mask off areas that won't be surface treated. Here's how the surface treatment is done. The panels that pass inspection are processed through a chemical bath. The chemistry grows oxide crystals on the surface of the copper to increase the surface area. This surface treatment enhances the strength of the bond during lamination, improving overall board reliability. Flexible circuits may require a specific formulation of oxide. When double treat copper is used, surface treatment is unnecessary. After the surface treatment process, the panels are normally oven dried at a low temperature to remove any moisture from the innerlayers. At this point the innerlayers are ready for the lamination operation. We'll be building up a sandwich of conductive layers -- which will be separated by insulating layers of resin coated fiberglass, called prepreg, or with adhesives. Once again we'll be using the tooling holes to align all of the layers. The tooling pins are pressed into the tooling fixture, then a steel separator plate is placed onto the pins. The separator plate protects the outer surfaces from indentations. Now a release sheet is added -- to keep any squeezed out resin from adhering to the metal plate. Heat lagging material may also be added. Next, we place one sheet of copper foil, or copper laminate onto the tooling pins. This will eventually become our outer conductive pattern, or layer one. A specific number of sheets of prepreg, or adhesive are now added. Then we're ready to add one of the fully processed double sided innerlayers onto the sandwich. This will become layers 2 and 3. Then we add the required number of sheets of prepreg, or adhesive -- followed by another innerlayer core -- which will be layers 4 and 5. This process continues until the specific number of layers are completed. The final layer will be made out of another sheet of copper foil, or copper laminate. A release sheet is then placed on top, followed by another steel separator plate to finish off the stack. Now we can repeat this exact lay-up several times to get as many multilayer flexible circuits into the lamination press as possible -- since the lamination process 6

7 can take over an hour to complete. The multilayer flexible circuits are now laminated together under heat and pressure. The heat of the lamination press causes the partially cured prepreg resin, or adhesive to melt and flow. Vacuum pressure is normally used to evacuate the air that's in between all of the materials. The prepreg, or adhesive then hardens or gels to bond all of the layers together. After the resin has hardened and cured, the flexible circuits are removed and cooled. The lamination fixtures are now disassembled. The next step is to check the thickness of the laminated panels. This thickness measurement will determine if anything has gone wrong during the lamination procedure. Once the desired thickness is verified, the panels may be baked once again -- to relieve any stresses created during the lamination process. At this point we're ready to begin processing the plated through hole layer to layer interconnections and the outerlayers. This occurs in the same manner as a double sided board. After drilling, multilayer flexible circuits require an etch back process to the innerlayer copper lands. The fabrication process continues. Now the individual flexible circuits are ready to be removed from the fabrication panel. This is true whether the panels are single sided, doubled sided or multilayer. There are several ways to do this. In many cases, the circuits are removed from the panel with a steel rule die. The die is placed on a press table and the circuit is cut out with a die blade. Sometimes the flexible circuits are cut almost all the way out -- except for a few notches -- to keep the circuits attached to the panel. This method keeps the individual circuits in panel format -- to simplify handling during further processing. A punch press can also be used to remove the flex circuits from the panels. For very small quantities - - a hand cutting template or pattern can be used to shape the circuits. Before or after the flexible circuits are removed from the panel, there may be additional finishing operations. Stiffeners may be added for support areas. Adding stiffeners is a critical and labor intensive process. There are many varieties of stiffeners that might be used depending on the application. The individual flexible circuits may then be electrically tested for opens or shorts. Electrical testing will insure that the circuits function properly -- before any of the electronic components are attached. The flexible circuits may also undergo a final cleaning and testing procedure to remove ionic or electrically conductive contaminantion from the surface. At this point there may also be a final visual inspection -- to make sure there aren't any unacceptable defects. Some reliability testing may be done following final inspection depending on the 7

8 circuit application. This may include life, stress and evironmental testing. The final step is the packaging and shipment to the customer -- or component assembler. Packaging materials, methods and storage time can all affect the integrity of the final product. Cellular phones, lap top computers, cameras and camcoders -- we rely on these and other electronic products every day. Every flexible circuit you build is used by someone -- to make their life more productive and enjoyable. Dependability of the finished product is everyone's responsibility. From the first fabrication step to the last. 8