Circuit Boards—Green Residue Always Means Corrosion--Or Does It?

Based on an original publication by Alpha

The automatic thinking would be that if you see green residue on a circuit board, you’re going to see corrosion underneath. Depending on how serious, corrosion can cause electrical reliability problems.

Commercial water-white rosin is approximately 80% abietic acid and 20% organic acids and doesn’t enable reaction of soldering. When heated, abietic acids combine with the oxide on the copper surface creating copper abietic very similar looking to the corrosive products of copper.

Rosin fluxes have small quantities of organic activating agents that cause rosin copper oxide to react and result in better soldering. The activators do not cause corrosion; however they do combine with the copper oxide and form green copper abietate. Unfortunately, this green residue cannot easily be distinguished from the corrosion products of copper.

A study that tested heat and humidity using coupons suggested that certain green copper/flux residues do not always cause or mean corrosion or definite decreased circuit reliability. Even though some liquid and solder paste fluxes showed high SIR values in testing and left green residues on copper surfaces after soldering, heat exposure or high humidity—the copper surfaces beneath the residue appeared shiny and not pitted and—not corroded. However, some green residues do indicate corrosion, but not always.

 

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