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Technical Trends - A Primer on Lead-Free Rework
with Bryant Underwood
Reverse Logistics Magazine, Aug/Sept 2007
If you are doing any soldering based repair work you must address the need for properly supporting your Clients' need for lead-free compliance. For most, getting their "operational arms" around the need for lead-free rework can seem like a very daunting task. This month we intend to equip you with a solid basis to understand the issues around lead-free repair and a framework to address the basic lead-free rework needs.
Background and Issues:
The chief component in most lead-free solders is tin and tin has some negative behaviors to be aware of. Legend is that in 1812 when Napoleon's army got bogged down in Russia, the intense winter was to blame. Some commentators point to more than just the winter's cold on bones as the problem. The real problem many feel was tin. The shiny buttons on the soldiers' uniforms were made of tin and when tin is exposed to temperatures below 56 degrees Fahrenheit it begins to degrade and turn to powder. The result is that the army was reduced to groping for ways to keep clothing closed and for staying warm rather than fighting. The characteristic for tin to turn to powder in the cold is called "Tin Plague." In addition, tin is also corrosive.
Another point to understanding the challenge in performing lead-free rework involves a chart called the 'Galvanic Series.' In this chart (see Mil Std-889), metals are sorted as being on a scale that ranges from Anodic/active or Cathodic/noble. When similar metals are used in an alloy it will generally be stable, but when the alloy contains metals from extreme positions on the scale there are usually corrosion and stability risks. Tin and lead are metals that are on the Anodic side of this scale with similar characteristics. This allows 60/40 lead type solders to be stable and easy to rework. When lead is removed from the alloy it is often replaced with a mixture of copper and silver. Since copper is in the middle of the galvanic scale and silver is all the way at the other noble end of the scale, this is an alloy as a solder that has some tough issues to overcome. This type of solder alloy with Tin-Silver-Copper is typically referred to as an 'SAC' type solder and is the most common rework type for lead-free repair.
If the time that a heated solder joint changes from liquid to solid (called the plastic phase) is lengthy then there is a greater likelihood that the joint will be damaged. With the standard 60/40 lead solder there is a very narrow plastic phase allowing the solder to become solid very quickly. When a replacement alloy formulation for lead-free solder is developed the engineer seeks to create a mixture that is "eutectic." This term refers to the alloy's ability to melt at a lower temperature and have a very narrow time when the mixture is 'plastic.' For lead-free solders this is a much tougher behavior to produce. As a result lead-free repair is performed at higher temperatures and is much more prone to a rough surface appearance.
I have a friend that runs a reflow production line building military equipment. The greatest fears of their engineering team regarding lead-free solder implementations is a problem called "tin whiskers." Lead-free solder has a very strange behavior that over time (some data also indicates current and temperature can promote this) the alloy will grow whiskers that reach out in all directions. This creates huge short circuit risks, especially for modern circuitry with such tight lead spacing.
The rough surface appearance of lead-free solder is another concern with successful rework and repair. The visual change with lead-free inspection is drastic. To the point that almost no element of lead type solder visual inspection can be used in lead-free. For example in lead based solder inspection, "copper halos" will typically cause a rejection but occur normally with lead-free processes. The situation can be greatly improved by performing the rework with nitrogen. The result will be better wetting and a more shiny appearance that is much more 'inspectable.'
Most flux formulas for lead solder use RMA (rosin mildly activated) type fluxes. These are alcohol based and although they work well with lead based solders the high temperatures needed for lead-free make these flux types unsuitable for lead-free. No-clean water based flux with 4% solids are good starting points for lead-free rework. Many also report good results with the newer gelled flux formulations. Regardless, if you do not use a no-clean type flux, the high temperatures needed for lead-free will yield a residue that is much tougher to clean.
Recommendations:
- Review and understand the documentation standards that our already out in the public domain. Some key documents to begin with:
- (Overview) http://rohs-wee.org/guidance.htm
- (Tin Whiskers) JESD22A121
- (Alloy Type Identification) JESD97, IPC-1066
- (Galvanic Series) MIL-Std-889
- (Alloy Formulations) http://www.europeanleadfree.net/
- (Rework Guide) NPL Report Matc(A)106
- (Inspection) J-STD-02B/C, IEC 60068-2
- Get your staff certified with an IPC level course.
- Upgrade your BGA capability. The higher temperatures for lead-free demand thermal profiling and tight mechanical controls to prevent collateral damage during rework. That old hand held BGA machine will just not provide the needed productivity or quality your Client will expect.
- Consider using Nitrogen. The process will be easier to inspect if you use nitrogen. To mitigate the cost consider using a "nitrogen farm" instead of gas from tanks. Nitrogen farms produce the gas by using filters to extract nitrogen directly, from the air. The costs are reasonable compared to the quality benefits.
- For solder iron based rework, use irons with standby settings that support longer tip life and fast thermal control/sinking for higher productivity and better joints.
- Select high quality (yes, they will cost more) solder iron tips. Remember from above, that tin is corrosive and the higher tin content and temperatures from lead-free will greatly shorten tip life. Poor tips just slow down your staff and cause damaged boards.
- Research your vendors part numbering identification schemes to ensure the parts you are using for replacement are compatible with the boards you are reworking. Very often you may have "new old stock" that is tinned with lead based solder and may not be usable.
- Understand the marking scheme (refer to JESD97) on the PCB and use this to help you select the right solder alloy. Once the alloy is dialed-in get a recommendation from the solder vendor or Client for the right flux.
- Beware of mixing lead and lead-free solder. Some combinations will have a very long plastic phase and almost guarantee a failed joint. In addition you would have just voided the lead-free marking on the PCB.
- Build procedures that limit what staff can correct a solder error on a reworked PCB. The high temperatures may only allow one more chance to get it right, having rules for the escalation to highly skilled operators can greatly reduce overworking and destroying a PCB.
The conversion is not easy, but bridging the gap will help differentiate your service offering and help your Client comply with the global regulations.
