Soldering bath contamination sources
PCB surface finish, component leads, and machine design are the most common contamination sources for lead-free solder baths.
Tony Lentz of the FCT Assembly lists the Core Pollution Sources Catalog. The most common sources of solder bath contamination (both wave and selectivity) are metals from PCBs and components. For example, if the boards are coated with a tin-lead HASL finish and are assembled in a lead-free regime, the lead will dissolve from the HASL coating and penetrate into the solder bath. On the other hand, if the boards are covered with a OSP layer on copper, the copper from the pads will dissolve and mix with the solder bath as well.
In turn, the leads of the components are often tin or nickel plated, which may also dissolve in the solder bath.
A much rarer case is that you mistakenly add the wrong solder rod to the crucible: for example, if a tin-lead solder bar is added to a lead-free solder bath, lead contamination is likely to rise above the RoHS limit (0.1 wt%).
Kevin Mobley of the General Atomics Electromagnetic Systems Group slightly extends the catalog of contaminants from PCB surface finish to gold and silver. In addition, metals such as cadmium, zinc and aluminum can infiltrate the crucible from the brazing equipment such as various types of frames, clamps, tips, etc. Virtually anything that is used in the wave brazing process can release contaminants into the crucible, although they are usually that's a very small amount. As another expert quoted later in this article points out, machine and fixture parts are also a source of iron contamination.
Kevin Mobley also notes that most solder suppliers provide their customers with the service of analyzing the composition of the solder crucible. In most cases, if a problem occurs, the supplier may recommend the use of certain additives to the crucible that will melt the contaminants and remove them as slag, but in some cases it may be necessary to replace the entire contents of the crucible.
Carlos Bouras of Nordson Select goes deeper into the topic of crucible cleanliness analysis: in general, it is recommended that solder baths containing lead-free alloys be analyzed once every 3-6 months or every 8,000 plates assembled, whichever comes first. Although full analysis is usually performed, solder analysis primarily focuses on the three main metallic impurities: copper (Cu), lead (Pb), and iron (Fe).
As mentioned earlier, the main source of copper contamination is copper dissolution from the PCB surface finish that forms the copper-tin intermetallic compound (CuSn). Copper contamination is tolerated up to 1.0% by weight, and contamination levels greater than 1.0% may slow solder flow and adversely affect machine performance.
The upper level of lead contamination, while still being RoHS compliant, is 0.1%. The lead infiltrating the solder crucible promotes the formation of lead-rich bath fractions with a lower melting point in the solder joints, which can lead to potential cracks and other hidden defects.
Iron contamination is tolerated up to a maximum of 0.02% and is primarily due to the leaching of machine components in contact with tin-rich lead-free solders which form the iron-tin intermetallic compound (FeSn2). Iron contamination levels in excess of 0.02% can potentially make the solder joints brittle.
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