Assembly of Flexible Circuits with Lead-Free Solder Alloy

Transcription

1 Assembly of Flexible Circuits with Lead-Free Solder Alloy Bob Willis bobwillisonline.com Your Delegate Webinar Control Panel Open and close your panel Full screen view Raise hand for Q&A at the end Submit text questions during or at the end 2

2 Bob Willis Involvement in Lead-Free Process Development Bob Willis has been involved with the introduction and implementation of lead-free process technology for the last seven years. He received A SOLDERTEC/Tin Technology Global Lead-Free Award for his contribution to the industry, helping implementation of the technology. Bob has been a monthly contributor to Global SMT magazine for the last six years. He was responsible for co-ordination and introduction of the First series of hands-on lead-free training workshops in Europe for Cookson Electronics during These events were run in France, Italy and the UK and involved lead-free theory, hands-on paste printing, reflow, wave and hand soldering exercises. Each non commercial event provided the first opportunity for engineers to get first hand experience in the use of lead-free production processes and money raised from the events was presented to local charity. More recently he co-ordinated the SMART Group Lead-Free Hands On Experience at Nepcon Electronics This gave the opportunity for over 150 engineers to process four different PCB solder finishes, with two different lead-free pastes through convection and vapour phase reflow. He also organised Lead-Free Experience 2, in He has also run training workshops with research groups like ITTF, SINTEF, NPL & IVF in Europe. Bob has organised and run three lead-free production lines at international exhibitions Productronica, Hanover Fair and Nepcon Electronics in Germany and England to provide an insight to the practical use of lead-free soldering on BGA Ball Grid Array, CSP Chip Scale Package, 0210 chip and through hole intrusive reflow connectors. This resulted in many technical papers being published in Germany, USA and the United Kingdom. Bob also defined the process and assisted with the set-up and running of the first Simultaneous Double Sided Lead-Free Reflow process using tin/silver/copper for reflow of through hole and surface mount products. Bob also had the pleasure of contributing a small section to the first Lead-Free Soldering text book Environment - Friendly Electronics: Lead-Free Technology written by Jennie Hwang in The section provided examples of the type of lead-free defects companies may experience in production. Further illustrations of lead-free joints have been featured in here most recent publication Implementing Lead-Free Electronics He has helped produce booklets on x-ray inspection and lead-free defects with DAGE Industries, Balver Zinn and SMART Group Mr Willis led the SMART Group Lead-Free Mission to Japan and with this team produced a report and organised several conference presentations on their findings. The mission was supported by the DTI and visited many companies in Japan as well as presenting a seminar in Tokyo at the British Embassy to over 60 technologists and senior managers of many of Japans leading producers. Bob was responsible for the Lead-Free Assembly & Soldering "CookBook" CD-ROM concept in 1999, the world s first interactive training resource. He implemented the concept and produced the interactive CD in partnership with the National Physical Laboratory (NPL), drawing on the many resources available in the industry including valuable work from NPL and the DTI. This incorporated many interviews with leading engineers involved with lead-free research and process introduction; the CD-ROM is now in its 3rd edition. Flexible Circuit Text Books & CD

3 Assembly of Flexible Circuits with Lead-Free Solder Alloy IPC World PCB Production Report 2010 IPC members report not published as yet

4 IPC World PCB Production Report 2010 IPC World PCB Production Report 2010

5 Flexible Assembly Options Tin/lead of Lead-Free Soldering Convection, vapour phase & conduction Low temperature solder alloys Press Fit (Flex-Rigid) Conductive Adhesives Wire Bonding Flip Chip Assembly TAB Tape Automated Bonding Flexible Circuit Materials Polyester soldering < 150oC Polyomide (Kapton) <300oC Baking circuits prior to assembly 1-2hrs o C Refer to IPC 1601 Storage and Baking Guide

6 Flexible Circuit Assembly Flexible Circuit Assembly

7 Flexible Circuit Construction Mask defined pads or part of the pad extends under the coverlay or liquid mask Flexible Circuit Construction

8 Tape Automated Bonding Flexible Circuit Assembly Assembly of two flexible circuits during November 2005 and M ay 2006 on product lines at international exhibitions

9 Flexible Circuit Construction Base M aterial mm Copper 18/18 um ESPANEX from Holders Technologies Liquid Solder M ask - M PR80 Amber - Nippon Steel Solder Finishes Immersion Nickel/Gold - Aurotech, from Atotech Immersion Tin - Stannatech from Atotech Immersion Silver - Sterling from M acdermid OSP - Glicoat SMD P2 from Shikoku Supporting Flexible Circuits Kapton sticky tape on corners Pressure sensitive low tack tape Magnets Clips Vacuum plates

10 Assembly Process Stages Germany Screen Printing - Speedprint SP 200/TECAN Stencils Solder Paste - Kester Placement - JUKI Automation Reflow Essemtec Profiling - KIC Automatical Optical Inspection - OM RON Electronics X-ray inspection - Dage X-Ray Systems Optical Inspection - Vision Engineering Rework - Pace Europe Rework M aterials Henkel Loctite Flexible Circuit Construction Wetting indic ator str ips for solder paste dot solder ability patter n SOT23 component positions SOIC16 component positions 0402 chip component positions

11 Flexible Circuit Reflow The gif animation was produced fromvideo images taken during filming the reflow of a QFN package on a flexible circuit. Movement can be related to the flexible or due to air movement in the reflow process Pallet and Flex Circuit Paste being printed/reflowed in side this area of the pallet

12 Assembly Pallet for Flexible Circuit Thermal relief areas on the base of pallet to improve delta T during reflow. Also consider drain holes under the flex if using vapour phase for lead-free reflow. Pallets from Schnaidt GmbH Assembly Pallet for Flexible Circuit Double sided flexible circuit on assembly pallet. Fixed and sprung pallets location pins used for the flexible. Protrusion of pins above the circuit was approximately Pallets from Schnaidt GmbH

13 Assembly Pallet for Flexible Circuit Circuits do lift on the edge of the tooling holes which does not cause a problem during assembly. This may be due to the copper being relieved around the hole causing the flex film to distort when clamped. Printing of Lead-Free Solder Paste The (150um) stencil was recessed on the base of the foil to allow for any protrusion of the pallet clips. The stencils was recessed by The stencil could be reduced to (125um)

14 Printing of Lead-Free Solder Paste Printing of lead-free solder paste with a (150um)stencil. Two different aperture sizes were used on three different component footprints. Specific design rules used on the solderability wetting strips. Recesses were placed around pin protrusions from the pallet. Component Placement on Flexible Circuit Placement of components on 0402, SOT23 and SOIC16 paste deposits. LGA Land Grid Array or QFN to be used in the Lead-Free Experience 4 and at Productronica, Germany

15 Flexible Board and Pallet Profile Locations Air temperature 5 Pallet temperature on the bottom 3 Reflow Profile of Circuit on Pallet

16 Temperature Profiling Flexible Assembly Pallet for Flexible Circuit Modified flexible pallet with deeper recess in the areas of the circuits and solderability test strips. Drain holes for fluid are positioned under the flexible when using vapour phase reflow soldering Pallets from Schnaidt GmbH

17 Temperature Profiling Flexible Vapour Phase Reflow Profile

18 Reflow Oven Parameters Zone Number: Top Set points (deg C): Bottom Set points: Conveyor Speed 39.5 cm/minute Product Name: Process Window Name: Oven Name: Flexible Circuit Sn/Ag/Cu RO400FC Example Lead-Free Joints Sn/Ag/Cu solder joints after convection reflow in air. The paste alloy was Sn96/Ag3/Cu05. Its was a type 4 powder also being used on another rigid board design for

19 X-Ray Examination of Joints Images show voiding due to the time in a liquid state. The speed of reflow and the time above liquidus temperature was related to the pallet mass. Incomplete reflow of wetting indicators due to non relief of pallet under strips Flexible Circuit Design Four PCB solder finishes, gold, silver, copper OSP, Tin & one flexible foil type provided by Holders Technology, standard photo resist

20 Assembly Process Used Speedline Screen Printer Siemens Component Placement SMT Convection Reflow BTU Convection Reflow AD Automation Vapour Phase Reflow Pace Hand Soldering/SMT Rework Vision Engineering Inspection Solderstar Temperature Profiling DAGE X-ray inspection Systegration XRF Testing Marantz AOI Christian Koenen Stencils Exception PCB Henkel Loctite Solder Paste Printing Images Solder Paste: M ulticore LF318 (97SCLF318AGS89V)

21 Lead-Free Reflow Soldering Conv ection BTU Py ramax 98 7 zones 60cm/min Vapour Phase A&D Automation SLC 504 (Graph 0084) 74 sec abov e 220oC, 27 sec above 230oC Solder Paste: Multicore LF318 (97SCLF318AGS89V) Lead-Free Reflow Soldering Convection BTU Pyramax 98 7 zones 60cm/min Vapour Phase A&D Automation SLC 504 (Graph 0084) 74 sec above 220 o C, 27 sec above 230 o C Solder Paste: Multicore LF318 (97SCLF318AGS89V)

22 New Flexible Pallet Design Pallet with holes to help drain VPS fluid during reflow, these also in theory help heat transfer to the flexible design area on the circuit New Flexible Pallet Design Further modifications on hole size required for vapour phase reflow to reduce fluid trapping and drainage. This will also benefit the throughput speed for fixtures.

23 Solderability Solder Paste Dot Test The solderability test strip on both ends of the flexible and one end of the rigid board contains square apertures on a descending pitch. Separation between the apertures each step. Tests were conducted one and two reflows. Wetting balance measurements are also being conducted on the rigid board test coupon Solderability Solder Paste Dot Test Images from NPL Lead-Free Masterclass Workshops

24 Solderability Solder Paste Dot Test OSP Silver Gold Tin Half of the flex showing test area and one section populated with components. Lead-Free Solder Spot Test Pattern Solderability testing with lead-free solder paste printed with a stencil then reflowed

25 Inspection of Land Grid Array LGAs The LGA have been separated in two different ways One is saw singulated and the other punched. The saw singulated are square. The punch singulated have chamfered corners, this does have an impact on the way the solder wets the side termination if it wets at all. Inspection of Land Grid Array LGAs Its clear from this view that solder joints have formed on the base of the device, if the side termination do not wet then the visual appearance of the joints may vary making it difficult for AOI inspection. The land pattern should still ex tend outside of the component footprint and the length of the pad designed under the part.

26 Wetting to Land Grid Array Centre Pads OSP Silver Gold Tin Depending on the wetting and coverage of the pads when the lead-free solder reflows, LGA components may float above the termination pads leaving open joints. Results after second reflow LGA Component Floating Examples of component float seen duringx-ray inspection of flexible circuits. Reducing the amount of voiding, checking the impact of the surface finish or the stencil design may reduce the floating

27 Lead-Free Process Defect Experience The reflow results in terms of wetting were superior on vapour phase, however we did experience tombstoning of 0402 chip capacitors. Convection reflow the solderability was satisfactory, however there was evidence of LGAs floating on the pads affected by the wetting of the PCB solder finish. Sticky Assembly Pallet for Flexible Circuit Flexible Assembly Sequence Place the removal plate on the fixture with alignment pins Place the FPC on the removal plate and fixture. Place the Magic Resin Carrier on FPC, removal plate and fixture. Remove the fixture with alignment pins. Turn over Take off the removal plate. Complete the FPC setting on Magic Resin Carrier Pass through the SMT mounting machine. Remove the FPC from Magic Resin carrier after re-flow soldering Alternative pallet concept for high/medium volume flex circuits uses a tacky surface with no tooling pins. Suggested to exceed 500 assembly cycles and available from SENJU.

28 Separating Flexible Circuits Die punching has been traditionally used for separating flexible circuits from a multi. Laser is an alternative technique that reduces the need for component clearances. Video from LPKF Conduction Reflow Soldering Contact heating for polyimide flex ible circuits on black belt reflow system, each assembly is in contact with the belt which passes over the hot plates to reflow the solder paste

29 Hot Plate Reflow Cooling Plate Hot Plate Placement Plate Reflow of flexible circuits with solder levelled pads. Pads are flux ed, components manually placed and then slit onto a hot plate. After reflow the circuit is slid off the plate to a cooling plate. The flex circuits featured one PLCC and four chip components. The circuits were then immersion cleaned. Remember that circuits will tend to float during immersion cleaning or fly on inline systems Wave Soldering The flex ible must be produced as a multiple circuit with rigidising or placed in a support pallet just like a selective pallet for double sided soldering and through hole parts. The wave solder locations are the only areas ex posed to the solder wave

30 Reel to Reel Assembly Examples featuring DEK, Siemens & Rehm Reel to Reel Fabrication Example reel to reel fabrication of the flexible circuits at Teknoflex Ltd, Chichester, England

31 Rework of Flexible Circuits with Lead-Free Rework joints with hot air pencil Use flux prior to reflow Remove and replace components Set-point on tool o C Use lead-free wire to set-up air pencil distance from joints for reflow Removal of solder shorts with solder wick Shorts purposely added to SOIC16 leads for testing Set-point on iron 380 o C No issues of delamination or pad lift on parts after two rework operations. Pad dimensions for the parts used could be reduced to improve use of the surface area but adhesion of the pads would be reduced. Modifications can be made, but only when this is really necessary Rework of Flexible Circuits with Lead-Free Video clip showing the removal of SOT23 with lead-free alloy SnAgCu from the surface of a flexible circuit. Flux is applied first to aid removal followed but reflow with hot air pencil.

32 Rework of Flexible Circuits with Lead-Free LGA are reworked in the same way for removal using a hot air pencil. Replacement of a new part would require the ability to align the part correctly prior to reflow. Preheating the flex on a hot plate did speed up reflow but is not necessary Rework of Flexible Circuits with Lead-Free Video clip showing the removal of SOIC with lead-free alloy SnAgCu from the surface of a flexible circuit. Flux is applied first to aid removal followed buy reflow with hot air pencil

33 Lead-Free Experience Report 150 page r epor t on Introduction of lead-free Do you have any questions? bobwillisonline.com