Copper erosion and bath contamination: when to change the alloy
A solder bath does not stay the same: with every board it dissolves a little copper, picks up iron from the machine parts and other metals from surface finishes. Over time the composition changes and the joints deteriorate. This guide explains which contaminants matter, what effects they have and how to decide when to act.
Dickmann Srl · updated
Where contamination comes from
| Contaminant | Where it comes from | Main effect |
|---|---|---|
| Copper (Cu) | Pads, leads and plated through holes of the boards; in HASL, large areas of bare copper | Raises the liquidus, makes the alloy less fluid: more bridging and icicles |
| Iron (Fe) | Solder pot, pump and nozzles of the machine, attacked more by lead-free alloys | Hard compounds that impair wettability and joint appearance |
| Gold (Au) | Gold finishes on components and boards | Brittle compounds in the joints |
| Zinc, aluminium | Metal parts and accidental contamination | More dross and poorer wettability even in small quantities |
| Lead (Pb) | Components with lead-bearing finishes in a lead-free bath | The alloy may no longer be RoHS compliant |
Copper erosion
Molten tin dissolves copper: it is the same phenomenon that creates the bond between alloy and pad, but in the bath the copper builds up. Lead-free alloys, richer in tin and hotter, dissolve copper faster than tin-lead alloys, and selective soldering, with small nozzles and long contact times, can even erode the board's pads.
Alloys with nickel, such as 99SnCuNiGe, slow down copper dissolution compared with tin-copper alloys without nickel. For a comparison of alloys see SAC305, SAC0307 or 99SnCuNiGe.
When to act
Contamination limits for solder baths are given in the IPC J-STD-001 standard and in the recommendations of the alloy and machine manufacturers. The practical criterion is to compare the bath analysis with these limits: when a contaminant approaches the threshold, the bath must be diluted with fresh alloy or partly replaced.
Signs that the bath should be analysed before the scheduled date: more bridging and icicles, unfilled holes with unchanged parameters, dull or grainy joints, more dross than usual.
- Set an analysis interval based on volumes and the boards produced, and stick to it.
- Record the results: the trend of copper over time tells you more than a single analysis.
- Always top up with the same alloy as the bath.
- After a dilution or an alloy change, repeat the analysis.
Dickmann bath analysis
Dickmann analyses customers' solder baths: just send a sample of at least 250 grams of alloy and you receive an impurity report with recommendations for correcting them. See our analysis and recovery services. For topping up, we offer wave soldering bars, solid wire for selective soldering and HASL alloys.