SMART Group

As in previous years SMART Group and EMP magazine have combined to provide the industry a practical solution to lead-free. For the last three years we have made the understanding of the lead-free transition as easy to follow as possible with the Lead-Free Experience. Engineers were able to see and use production equipment, utilising lead-free alternative alloys on board designs featuring ball grid array, surface mount, 0201 & 01005 chips and through-hole reflow terminations. They were able to witness and discuss many of the issues raised by one of the most significant industry challenges in many years recalled Bob Wills - "Mr Lead-Free Experience" on this year's event.

Delegates were also encouraged to bring board designs and process issues and have all their questions answered by members of the SMART Group Technical Committee, who have now had a number of years practical experience with lead-free production.

The SMART Group Lead-Free Hands On Experience 3 ran both days of the exhibition providing delegates with the opportunity to visit Nepcon and be first with lead-free process solutions. SMART Group now offers the largest collection of training, inspection and education material on lead-free assembly and soldering anywhere in the world.

The Experience also gave the opportunity for engineers to find out about LEADOUT, one of Europe's biggest collaborative projects on lead-free technology. SMART Group is a key member of the project and all the data from the SMART Group Experience will form part of a technical review for LEADOUT member companies.

So what were some of the key aspects of this years Experience? A full report is due in a couple of months so look out for the announcements on the web site www.smartgroup.org

Components

Key considerations for component compatibility with lead-free solders are temperature limits and termination finish - both can have an impact on the process and reliability. Lead contamination in terminations can lead to secondary reflow during assembly. To date many surface mount parts have 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 still 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.

Example of 01005 chip capacitor component prior to reflow soldering and one example after successful reflow of 01005 in vapour phase reflow

Printed Boards

During Experience 3 engineers were able to solder PCBs with many of the alterative tin/lead solder finishes. Boards were produced with two different laminates and four 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% 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. Based on the results of the trials conducted during the event the solder finishes would be ranked in the following order of solderability and wetting: first would be gold then silver, tin and copper OSP. Reliability of through hole vias was again a focus with special test patterns incorporated into the Experience boards.

The X-ray image shows the 0.2mm test patterns included in all the test boards used in the Experience. A microsection is also shown of selected vias after reflow simulations. After multiple testing through reflow, wave and rework there was no failure on standard FR4 laminate or high Tg versions of the board.

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. The assembly of the components on each different finish did not pose or raise any issues during the Experience.

Solder Paste Printing

Solder paste printing should not be affected by the change to lead-free materials, which can
print fine pitches, 01005 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, increased their stencil apertures even though the latest version of the world's best known inspection criteria IPC610D suggests that incomplete coverage is only a process indicator and rightly so!

Lead-free solder paste may be slightly more expensive than tin/lead materials and with over 20% 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.

Reflow 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 fundamental 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. Three years of running the Lead-Free Experience has allowed the board design rules to be honed to perfection.

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. Wave soldering was successfully featured in this year's Lead-Free Experience and provided confidence to this in attendance.

A combination of tin/copper alloy, VOC free flux provided excellent results on through hole fill and soldering of SOIC and SOT23 terminations. The solvent based solder mask used on the boards also resulted in a visually very clean appearance.

Examples of the satisfactory results that can be obtained when processing through hole reflow and BGA in a lead-free assembly process.

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. The key is the effective use of pre-heat to the base of the board for large packages and BGA..

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. This has also been confirmed by NEMI in their recent project work in the USA.

Inspection

Surface appearance and wetting behaviour of lead-free joints may differ from their tin/lead counterparts. 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. The latest version of IPC 610D covers lead-free termination and provides some criteria. SMART Group has its own inspection wall charts covering lead-free inspection for conventional, surface mount and ball grid array technology. The charts are available on-line at www.smartgroup.org in the smart-e-products section of the web site.

"Having run the SMART Group Lead-Free Experience for three years I must say it's great fun to experiment with different processes, materials and components and pass on the information to the industry. It is one of the most useful services SMART Group provides" commented Bob Willis at the end of this year's event.

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