SMART Group

SMART Group has been running successful lead-free events for the past five years. Now with the impending ban of tin/lead solders confirmed for July 2006 every company must have a solution. SMART provided the first Lead-Free Hands On Experience at Nepcon Electronics in June this provided a unique opportunity to gather practical experience with lead-free materials, assembly and process challenges. The lead-free feature was co-ordinated by members of the SMART Group’s technical committee and your columnist.

Delegates will be able to see and use production equipment with lead-free alternative alloys featuring surface mount and through hole terminations. They were able to witness many of the issues raised by what is the most significant change in our industry in many years. Delegates also brought board designs, process issues to have all their questions answered by two of the leading EMS providers in the industry, Sanmina-SCI and Solectron and UK’s leading lead-free research group National Physical Laboratory (NPL), each provided the technical presentations during the two events.

The Lead-Free Hands On Experience ran both days of the exhibition, with each session lasting approximately 4 hours per day, this still provided delegates with the opportunity to visit the show during their visit to sunny Brighton and be first with the lead-free solution.

So what are the lead-free challenges?

Printed Circuit Board

During the session engineers were 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% 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.

Care is needed when selecting the solder coating, the main issue is the long term solderability as nothing solders as well as solder. When boards are exposed to heat, as in adhesive curing or first side reflow subsequent reflow, hand soldering or wave soldering can be affected.

The finish used were:

Gold
Silver
OSP
Solder Levelled Tin/Nickel

Components

Key considerations for component compatibility with lead-free solders are the temperature limits and lead termination finish and both can have an impact on the process and reliability. To date many of the component finishes have not changed; however, alternatives exist like tin, silver, palladium and tin/copper/nickel. The main finish which will be widely adopted throughout the industry will be tin even though some people still have concerns with tin whiskers.

Changing component lead finishes is important to meet the requirements of the legislation, it is also important for joint reliability. The use of tin/lead plating can cause lead rich phases in the joints which can in term lead to failure of joints; changing to a lead-free alterative component finish prior to changing the solder alloy should not be an issue.

Traditionally tin/lead reflow is conducted at 215-225oC and wave solder between 245-255oC and all components have to meet these minimum temperature ranges to be suitable for assembly. With lead-free reflow being between 240-245oC and wave solder 260-270oC component specifications must change. These temperature ranges provide reference for specifying parts but only if the manufacturing processes are capable of controlling the processes within these tolerances.

The following components and finishes were used in the hands-on feature:

0201 Capacitor Sn on nickel electroplated
0603 Capacitor Sn on nickel "
0201 Resistor Sn on nickel "
0402 Resistor Sn on nickel "
0603 Resistor Sn on nickel "
SOT23 Transistor Sn3.0Ag0.5Cu Electroplated on FeNi lead frame
Surface mount LED- CuNiAu Electroplated
SOIC16 IC Sn2Cu electroplated on copper lead frame
All the above parts are qualified to 260C peak soldering temperature

In addition 280 tin/lead BGA and 400 Tin/Silver/Copper BGAs were used during the experience sessions

The three connectors were plated with lead/tin 90/10 finish although it is intended to make these parts available in tin finish for lead-free process in the future.

Connectors

96 way edge connector
20 pin header
9 way DIN

Screen Printing

Printing of solder paste should not be affected by a change to lead-free materials; they can be printed successfully to the finest pitch currently required. Experience has shown it can be used successfully for intrusive through hole reflow, 0201 and BGA assembly which will be included as part of the lead-free feature. Experience shows that lead-free alloy pastes do not wet or spread on the surface of the pad to the same degree as tin/lead. This can lead to the base material of the pad finish being exposed after reflow. To overcome this some companies have re-modified their stencil apertures to overcome this issue.

Printing can be one of the process stages where operators are currently exposed to lead in paste so there is a direct benefit to limit exposure on health grounds. The solder paste used may be slightly more expensive than tin/lead materials in the short term as it becomes readily accepted so waste should be kept to a minimum. With over 30% of paste scrapped after printing a sealed head printing process may be very beneficial in saving cost and maintaining the paste’s printing characteristics.

Two different stencil designs were used with two different lead-free paste each based on tin/silver/copper. Both stencils were 0.006” thick electroplated nickel, one was stepped down in the area of the 0201 chip components to 0.004”.

The printing processes can be affected by warped boards, hot or warm surfaces from previous reflow processes, all of which may occur as we move to lead-free assembly. Additionally as the pitch of the components gets smaller and the size of the process panels gets larger the tolerance of many boards becomes an issue. The stencil is produced from the design information and is extremely accurate; however, the board may not be due to expansion and contraction of the base materials used in industry.

Component Placement

Component placement machines today should be able to overcome most issues related to the change 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. This has certainly been an issue with mixed finishes where it was difficult to accurately recognise fiducial marks as people moved away from solder levelled surfaces.

Boards which may not be flat due to higher process temperatures with lead-free; this is not a placement problem but one the machines will need to deal with through effective use of board supports. As placement systems can overcome global and local PCB expansion and contraction issues to place parts the dimensional effects on the laminate should not be an issue.

Reflow Soldering

Many companies feel that reflow machines will need to change for lead-free but it depends on the technology level of system you currently have and the throughput speed required for your product. A professional oven with a four or five process zones and a process length of 3-4 metres should provide satisfactory results with higher temperatures. The only issue then is the exit temperature of the boards for the required throughput speed and the outer skin temperature of the oven from a health and safety point of view. If you have a higher reflow temperature then the exit temperature will be higher for the same conveyor speed unless additional cooling is provided.

Cooling can be very important to double sided boards with lead-free. It is inevitable that the exit temperature of the assembly will be higher; it may be 10-20 oC higher depending on the thermal mass of the assembly and its heat retention. If the boards are going to be sent directly into a printer for second side assembly with no buffer the boards can still be hot which can lead to printing defects.

Although not proven yet there may well be a relationship between cooling rates and the incidence of fillet lifting on through hole reflow parts. Just like with wave soldering, lifting of the solder fillet occurs when the solder joint interface is still in a liquid state.

It is also inevitable that machines will require more frequent maintenance of moving parts, conveyor links and convection fans due to higher temperatures. It is not clear yet what impact this will have in the future.

Wave Soldering

Lead-free will probably have more impact on the wave soldering process than any other process, many people were just not prepared for the issues which have become apparent. Alternative high tin content solders have a tendency to attack the stainless steel used in different parts of the ducting used to form the wave shape. This means that existing solder machines will need to be replaced or parts of the bath replaced. Fortunately most suppliers do have solutions to this equipment problem.

Generally the alternative lead-free alloys do tend to cause more shorts, this is due to the poor drainage of the solder from the base of the board. It can also be due to the learning curve which engineers are now going through to optimise processes. With the higher process temperatures and the common use of low residue fluxes there is a tendency for the flux to be removed from the board during preheat and initial wave contact which again can lead to shorts. This is another area which process engineering and suppliers will need to focus on.

Normally a solder bath will operate at 245-255 oC for tin/lead; in the case of common lead-free alloys these will be run at 260-270 oC which will increase the degree to which the boards will sag during solder wave contact. The use of board supports in the wave should be considered during board design, this requires a 3-4mm no go area in the centre of the board. An area is defined by the design engineer where no encroachment of surface mount or through hole terminations are allowed. A tensioned wire can then be positioned in the wave which supports the board as it passes through the wave. The support performance is far superior to the use of pallets or titanium clips.

Hand Soldering

Soldering through hole components will be conducted manually which will allow delegates to try out different alloys, PCB surface finishes and cored wire gauges. To date the only issue highlighted with hand soldering is the life of the solder bits. Some evidence suggests that when using lead-free alloys the bit plating is attacked leading to increase wear. Based on the corrosive nature of the high tin content alloys this may be the case but little testing has been conducted so far.

When using tips incorrectly without tinning them, leaving the soldering irons on for hours and using too higher temperature will of course have similar effect. The same situation may occur with de-soldering tools which basically have the same metallurgy and plating and need to be reviewed with your supplier.

De-soldering with copper braid/wick still works for removing 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 does deteriorate, this then impacts on the ability to wick solder quickly.

Surface Mount Rework and Repair

Surface mount rework is not difficult; the key is correct preparation and the use of the correct tools. Control of temperature is very important on small and large components which will mean more temperature profiling of boards as the process tolerances will be much tighter. Existing equipment will still be effective for lead-free rework, however the temperature set points may rise and some production supervisors may have to be a little more patience during component removal and replacement.

On joint reliability, concerns have been voiced on the use of mixed alloys during rework, if a combination of alloys was used would the joints be less reliable? 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 they explained the issues during the presentations with Solectron and Sanmina-SCI.

Visual Inspection

Lead-free alloys may have a different surface appearance; generally they do not seem to wet as well as their tin/lead counterparts. On surface mount pads solder paste does not spread as well during reflow, also in terms of wetting rise on surface mount terminations lead-free alloys do not seem to rise as high on terminations. The joints can still be judged based on existing international standards like IPC610 but it will be useful to have example lead-free alloy joints as reference to introduce inspectors to the different surface appearance.

X-ray is being increasing used for BGA, flip chip and through hole reflow. Some have debated the ability of the x-ray to work with lead-free alloys. It fair to say that you have need to make minor changes to setting but that is problem it. Many trials have shown that x-ray works and works as well as with traditional tin/lead alloys.

Most of the defects in conventional and surface mount assembly process can be seen with the alternative alloys and there will be an increase in solder shorts on the wave soldering process. The solder alloy does not seem to drain as well from the board during exit from the wave.

Lead-Free Hands On Experience Partners

The following companies helped to put the SMART Group Experience together:

Technology Partners

National Physical Laboratory www.npl.co.uk/ei/
Sanmina-SCI www.sanmina-sci.com
Solectron www.solectron.com

A&D Automation www.adauto.co.uk
Concoat www.concoat.co.uk
Contax www.contax.co.uk
Cookson Electronics www.cooksonelectronics.com
Dage www.dage-group.com
Flint Distribution www.flint.co.uk
Kaisertech www.kaisertech.com
Loctite Electronics www.loctite.com
Merlin Circuit Technology www.merlincircuit.co.uk
OK International www.okinternational.com
Pace www.paceworldwide.com
Purex International www.purexltd.co.uk
SMART Group www.smartgroup.org
SMT www.smt.uk.com
Speedprint www.speedprint-tech.co.uk
Tecan Componentes www.tecan.co.uk
Vision Engineering www.visioneng.com

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