SMART Group

With the impending ban of tin/lead solders confirmed for July 2006, every company must have a lead-free solution in place. To help EMP readers tackle this challenge, the SMART Group is provided the “Lead-Free Hands On Experience 2” at the Nepcon Brighton 2004 exhibition. Having hosted a number of lead-free events over the past five years, this second event will provided a unique opportunity to gather practical experience with lead-free materials, assembly processes and attend the UK's premier electronics exhibition.

Delegates were able to see and use production equipment, utilising lead-free alternative alloys on assemblies featuring ball grid array, surface mount, 0201 chip components and through-hole terminations. They were able to witness and discuss many of the issues raised by one of the most significant industry changes in many years. Delegates could also bring board designs and process issues and have all their questions answered by leading sub-contract partners, Solectron, Celestica and lead-free research group National Physical laboratory (NPL).

The SMART Group Lead-Free Hands On Experience 2 ran both days of the exhibition, with each session lasting around four hours. This provided delegates with the opportunity to visit the Nepcon Brighton exhibition and be first with lead-free process solutions.

All the data from the SMART Group Experience in 2003 and 2004 will form part of a technical review at the SMART Group European Conference in November so don’t miss out on the next ultimate Lead-Free Event. 16-18th November.

Components

Key considerations for component compatibility with lead-free solders are temperature limits and lead termination finish - both can have an impact on the process and reliability. Lead contamination in terminations can lead to joint failure and secondary reflow during assembly. To date many SMT parts have not changed, through hole parts tend to lag behind. Alternatives do exist like tin, palladium and gold. The main finish, which will be widely adopted throughout the industry, will be tin, although some concerns remain regarding tin whisker formation.

Traditionally tin/lead reflow is conducted at 215 to 225oC and wave solder between 245 and 255oC and all components have to meet these minimum temperature ranges to be suitable for assembly. With lead-free reflow between 230 and 240oC and wave solder at 260 to 265oC component specifications must change. Lead-free components are being supplied by ROHM and Practical Components. Through hole connectors supplied by Pheonix will be used to demonstrate intrusive reflow assembly during the Experience.

Printed Boards

During Experience 2 engineers will be able to solder PCBs with many of the alterative tin/lead solder finishes. This is one of the first considerations to address when changing to lead-free as much of the lead is contained in the PCB coating. Solder levelled boards still account for over 60 per cent of the market demand and tin/lead alloys are still used for levelling even though alternatives exist. Boards finished with gold, silver, organic solderable protector (OSP) and tin are some of the PCB finishes on offer to the industry and may be used as lead-free alternatives.

Component Placement

Component placement machines should be able to overcome most issues related to lead-free assembly. The move to different PCB finishes may require a close look at the vision capabilities
of older systems and some minor recalibration for gold, matt tin or copper OSP boards. Manufacturers may also find that boards are not always flat due to higher process temperatures.
Although this is not a placement problem, it is one that machines will need to deal with through effective use of board supports. Placement systems can overcome global and local PCB expansion and contraction issues, so the dimensional effects on laminate should not be an issue.

Solder Paste Printing

Solder paste printing should not be affected by the change to lead-free materials, which can
print fine pitches, 0201 and BGA devices as well as intrusive through hole boards. Research has shown that lead-free alloy pastes do not wet or spread on the surface of the pad to the same degree as tin/lead and this can lead to the base material of the pad finish being exposed after reflow. To overcome this, some companies have modified their stencil apertures.

Lead-free solder paste may be slightly more expensive than tin/lead materials and with over 30 per cent of paste scrapped after printing, a sealed head printing process could save costs and maintain printing characteristics. As the pitch of the components gets smaller and the size of the panels gets larger, board tolerance may also become an issue. Stencils produced from design information are accurate, however, the board may have warped due to expansion and contraction of the base materials leading to printing difficulties.

Soldering

Vapour phase reflow is becoming more popular again due to its ease of set-up, avoiding over heating on combinations of small and large boards. Lead-free has undoubtedly made engineers re-examine the fundamentals joint reflow and the benefits of a fixed peak temperature process which condensation reflow provides. Convection ovens with four or five process zones and a process length of three to four metres should provide satisfactory results with higher lead-free temperatures. Manufacturers will, however, need to consider the exit temperature of boards. Higher reflow temperatures will result in higher exit temperatures if the conveyor speed is not reduced. To maintain current conveyor speeds, additional cooling will be necessary. Machines will also require more frequent maintenance of moving parts, conveyor links and convection fans due to the higher temperatures.

Lead-free will have a greater impact on the wave soldering process as high tin content solders can attack the stainless steel parts of the wave bath. Fortunately most suppliers are working on solutions to this problem. Higher process temperatures combined with Iow residue fluxes, may mean that flux is removed during preheat, which leads to shorts. Engineers may also need to address board sagging caused by higher temperatures. Board supports can help and need to be considered during design.

Hand Soldering

Soldering through-hole components manually will allow delegates to try out different alloys, PCB surface finishes and cored wire gauges. To date the only issue with hand soldering is the life of the solder bit, as high tin content alloys may cause increased wear. The same issue may occur with de-soldering tools, which have the same metallurgy and plating and need to be reviewed with suppliers. De-soldering with copper braid/wick will remove solder shorts on surface mount and conventional through-hole terminations. Care should be taken with copper braids to make sure it is not stored for long periods as the solderability can deteriorate, impacting on the ability to wick solder quickly.

Lead-Free Rework

Lead-free surface mount rework should not cause difficulties - the 'key is correct preparation and use of the correct tools. Temperature control is important on small. and large components particularly BGA, which will mean more temperature profiling of boards, as process tolerances will be tighter.

Existing equipment will be effective for lead-free rework, however the temperature set points may rise and some production supervisors will have to be more patient during component removal and replacement. Joint reliability may also be a concern due to the use of mixed alloys during rework. Work done by the National Physical laboratory (NPL) has indicated that this may not be an issue with mixed lead-free materials and as one of Lead-Free Hands On Experience partners, NPL will be able to explain and discuss the issues in detail during the event.

Inspection

Surface appearance and wetting behaviour of lead-free joints may differ from their tin/lead counter parts. Solder paste spread on pads and wetting rise on surface mount terminations may not. match tin/lead performance. The joints can still be judged on existing international standards like IPC610, but example lead-free joints will introduce inspectors to the different surface appearance.

Most defects in the conventional and surface mount assembly process can be seen with the alternative alloys. There may be an increase in solder shorts on the wave soldering process as solder does not seem to drain as well from the board during exit from the wave. During the Lead-Free Hands On Experience, delegates will be able to examine boards with different alloys and compare the results for themselves.

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