|Ask the Experts|
November 12, 2020
Solder Contamination Sources
What are the most common sources of lead-free solder bath contamination?
|Expert Panel Responses|
The most likely sources of contamination in a solder bath (wave or selective) are metals from the circuit boards and/or components.
For example, if the boards are coated with tin/lead HASL finish and they are being run in a lead free wave solder system, then lead will dissolve from the HASL coating into the solder bath.
If the boards are coated with OSP over copper, then copper will dissolve from the board pads into the solder bath. Component leads are often coated with tin or nickel. Nickel from component leads can dissolve into the solder bath, increasing the overall nickel content.
In some cases the incorrect solder bar may be added to a solder pot by mistake. For example, if tin/lead solder bar is added to a lead free solder bath, then the lead contamination will likely rise over the RoHS limit (0.1% wt).
The most common sources of solder pot contamination will typically be material included in the assemblies that you are running across the wave. Copper can be a contaminate at certain levels and minute amounts strip off of each board that goes across the solder pot.
The same goes for gold or silver if you are using it as a surface finish. Cadmium, Zinc, and Aluminum can come of off solder fixtures, holding fixtures, terminals, lugs, etc. Almost anything you run across the wave will strip some material off, although it's normally very small quantities.
Most solder suppliers will provide solder pot analysis as a service to their customers. You take a sample from the solder pot, send it to them, and they do a complete analysis.
In most cases if there is an issue the supplier can recommend custom additions to the pot that will alloy with contaminates and take them off as dross, but in some cases replacement of the solder may be warranted.
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
It is generally recommended that solder baths containing lead-free alloys be analyzed once every 3-6 months or every 8,000 boards whichever occurs first. Although a complete analysis is typically preformed, a solder analysis is primarily focused on three main metallic contaminates: copper (Cu), lead (Pb) and iron (Fe) as well as other possible contaminants.
The primary source of copper contamination is the dissolution of copper from the circuit board surface finish which forms a copper-tin (CuSn) intermetallic. Copper contamination is tolerable up 1.0% by weight with contamination levels beyond 1.0% making the solder flow sluggish and adversely affect machine performance.
The upper level for lead contamination is 0.1% to ensure RoHS compliance. Despite operating with a lead-free solder bath, lead from tin-lead (SnPb) board surface finishes can leach into the solder pot resulting in lead rich low melting point segments within solder joints resulting in potential cracking and other hidden defects.
Iron contamination is tolerable up to a maximum amount is 0.02% and results primarily from the leaching of machine elements in contact with tin-rich lead-free solder alloys which forms an iron-tin (FeSn2) intermetallic. Levels of iron contamination in excess of 0.02% can potentially result in brittle solder joints and should be avoided.
We see generally the following while analysing solder
Technical Sales Manager
BLT Circuit Services Ltd
You have two major sources. First, your PCB. Due to the different surface finishes available you could add different types trace metals to your bath. For example, boards that have OSP as a surface finish could add copper contamination once the OSP dissolves. Second, the human factor. You could mistakenly add the wrong alloy to the solder pot therefore contaminating the process with lead.
Senior Manufacturing Engineer
Dross management can alter the ratio of tin/silver in a lead free alloy, and the Tin/Lead ratio. Attempting to recycle dross by separating metal from oxide is also a major source of contamination.
Lead free dross contains a higher ratio of tin (tin is less noble than silver). Tin lead dross contains a higher ratio of lead. Both effects can change the alloy content of the solder bath.
Global Director of Customer Technical Support
Alpha Assembly Solutions
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