The EMPF helpline received a call regarding the trace amounts of lead (Pb) allowable under the European Union RoHS directive. The EMPF was asked how the cumulative effect of lead leaching may affect the lead-free solder manufacturing processes and how lead-free electronics assemblies may become contaminated by the trace amounts of lead.

The European Union RoHS directive allows for materials to include trace amounts of lead (1000 ppm or 0.1% Pb). Lead contamination is one of the major lead-free soldering concerns. The three main pathways that lead may be introduced to the lead-free manufacturing process are component finishes, board finishes, and poor discipline within the manufacturing infrastructure.

Tin lead (SnPb) finished components and boards may be introduced via mistakes made by the design engineer, purchasing agent, and/or the distributor. On the parts list, the design engineers may not have called for lead-free finished components and boards. Purchasing may not be aware of lead-free purchasing issues. The distributor may not be disciplined enough to distinguish between SnPb and lead-free finished parts with the same part numbers.

During this period of transition to lead-free, several electronics manufacturers reported to the EMPF that they ordered lead-free components and instead received SnPb finished components. This mix-up is understandable because some component manufacturers are reluctant to issue new part numbers for components with lead-free finishes. Instead some component manufacturers track the lead-free parts by lot number, by package labels, and/or by adding a suffix to a pre-existing part number (PN). The suffix indicates that the part is lead-free or RoHS compliant.

Some component manufacturers reported to the EMPF their plans to stop using the PN suffix after converting all their production lines to lead-free. JEDEC-97 and IPC-1066 standards provide methodology to identify finishes by labels, but these standards are not being consistently followed by component manufacturers. A Certification of Compliance from a component manufacturer will not prevent an undisciplined distributor from mixing SnPb and lead-free finished parts with the same part numbers. If the design engineer, purchasing agent, or the distributor fails to call for and properly track lead-free parts, the likelihood increases of SnPb finished components inadvertently being introduced to a lead-free manufacturing process.

Poor discipline in maintaining equipment is another way in which lead may be introduced to lead free manufacturing. In hand soldering, a solder iron tip used for SnPb soldering may be used for lead free soldering. Cleaning the tip after SnPb hand soldering and before lead free hand soldering will not prevent lead contamination of the lead free solder joint. The EMPF recommends that solder iron tips used for lead-free soldering be kept separate from tips used in SnPb soldering.

For wave soldering, an operator may mistakenly put a SnPb solder bar into a lead-free solder pot. Two SnPb solder bars are enough to raise the lead content of an 800 pound lead-free solder pot over the 0.1% RoHS directive limit. Some solder manufacturers are voluntarily changing the shape of their lead-free bar stock, but its implementation is not universal. Visually, it is difficult to tell the differences between lead-free and SnPb bar stock, as per Figure 2-1. The most important strategy is to identify and control the inventory of SnPb and lead-free bar stock solder to prevent accidental solder pot lead contamination.

Wave solder pot maintenance becomes critical if a lead-free solder pot shares the wave solder system with a SnPb solder pot. Scrapping SnPb solder splash and fingers into the lead-free solder pot is another source of lead contamination if the solder pot was switched out between SnPb and lead-free runs.

Lead from SnPb finishes can leach into the lead-free solder pot during wave soldering. Over time, enough lead may leach to raise lead content in the solder pot over the 0.1% allowable by the RoHS directive. X-ray Fluorescence (XRF) analysis is a quick test method to confirm components have lead-free finishes.

To gain control of lead contamination in wave soldering, the EMPF recommends performing solder alloy analysis on a weekly basis to determine the rate at which the solder pot becomes contaminated. After 2 - 3 months of gathering data, the solder pot analysis may be reduced to once a month, depending on usage and the rate at which the solder pot is contaminated.
Lead contamination above the 0.1% RoHS limit is more a regulatory problem than a reliability problem. Reliability problems come into play when the lead contamination is significantly beyond the RoHS limit. Significant lead contamination depresses the melting point of the lead-free solder and there is a possibility that the lead may concentrate in the wrong area of the solder joint when the joint cools. This is especially a concern with SnPb components used on a lead free surface mount manufacturing line, especially with BGAs. SnPb BGA balls soldered with lead-free solders have an interconnection that is compromised, but the literature says this is a manufacturing issue, not a reliability issue. The reverse situation is worrisome for surface mount lines that have not converted to lead-free. When tin silver copper (SAC) BGA is soldered with SnPb and a SnPb profile, the SAC balls do not melt and collapse. The lead does not properly defuse into the SAC solder balls which causes embrittlement at the bottom of the solder balls near the pads on the boards.

Unless there is total control of the manufacturing processes and assurance that the lead is distributed evenly in the solder joint, mixing of lead and lead-free solders may result in an unreliable solder joint.

The EMPF offers Lead Free Manufacturing Training and analytical services geared towards resolving lead-free manufacturing issues. If you have any questions about Lead Free services available from the EMPF, or would like to inquire about our Lead Free Manufacturing training course, please call the Helpline at (610) 362-1320, or email us at helpline@empf.org.