Solder Paste Residue Corrosivity Assessment

Celine Puechagut, Anne-Marie Laugt, Emmanuelle Guene, Richard Anisko
Inventec Performance Chemicals
Bry sur Marne, France<

Lead free soldering with no clean solder pastes represent nowadays the most common process in electronic assembly. A solder paste is usually considered as no-clean if it passes all IPC J-STD-004 corrosion tests: copper mirror, copper panel corrosion test, Surface Insulation Resistance (SIR) and Elecrochemical Migration (ECM). Other SIR and ECM tests are described in Bellcore GR-78-CORE and JIS Z3197 standards.

Although SIR and ECM tests are recognized by all standards authorities to evaluate the solder paste residue corrosivity after reflow, a more selective method, the Bono test, has been developed and implemented in some French companies as a qualification criterion. It has been proven that compared to common corrosion tests, the Bono test better differentiates the nature of solder paste residues.

The Bono test differentiates the nature of solder paste residues better than other methods. Lead-free, no clean solder pastes have very different behaviours. Some can be very corrosive, as the Bono test proves even though common SIR and ECM tests fail to do so. Through the different experiments, it was shown that this method is consistent and repeatable. Even if the Fc value is low, the start of dendrite creation may be seen (since there are no short circuits between anodes and cathodes, the Fc value can be low), and a high Fc value does not necessarily lead to mousebites. Residue spreading is only a cosmetic issue, and halogens in the solder paste flux can cause copper dissolution.

The thermal profile will influence the solder paste residue corrosivity if the solder paste tends to be corrosive, however the reflow environment does not seem to have any influence. This test is also a good way to check the compatibility between solder pastes and conformal coatings, as it increases the difficulty to pass the test.

It should be noted that the Bono test procedure does not specify any real critical value above which a solder paste is considered to be corrosive. However, from the different experiments, a solder paste residue corrosivity assessment can be taken into account using the following guidelines: corrosivity does not exist in solder paste residue when the Fc value is under 2%(which can due to measurement precision), the solder paste residue corrosivity is acceptable when the Fc value is between 2 and 8%, and the solder paste residue starts to be corrosive when the Fc is above 8%.

In addition to characterizing solder paste residues, the Bono test can be performed for wave soldering fluxes, repair fluxes, and tacky fluxes using the same method and the same board.