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Solder Paste Residue Corrosivity Assessment
Solder Paste Residue Corrosivity Assessment
Although SIR and ECM tests are used to evaluate the solder paste residue corrosivity, a more selective method, the Bono test, has been developed.
Analysis Lab

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


Narration and Analysis:
Jim Hall
Jim Hall, ITM Consulting
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.
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Transcript
Although Surface Insulation Resistance (SIR) and the Electro-Chemical Migration (ECM) are recognized by all standards authorities as valid tests to evaluate the corrosivity of solder paste residue 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.

This paper describes the methodology, the material used, the specific test conditions, and the corrosion factor calculation for the Bono Test.

Tests performed consisted of a custom test board which was printed and reflowed, followed by 17 days exposure to an 85C/85%RH environment with a bias voltage of 20 VDC.

Daily resistance measurements were taken on each of the 10 identical cells on the board and the average % change in resistance calculated as a corrosion factor value which is representative of the corrosion effect and of the leakage current, caused by the flux residue.

Six solder paste formulations were evaluated using this test with some direct comparisons to the results of the standard SIR and ECM tests. The impacts of reflow profile and conformal coating were also evaluated.

The test results displayed significant differences in the corrosivity of the different individual paste residues.
Summary
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.
Conclusions
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.
Initially Published in the IPC Proceedings
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