Cleanliness test � �Area Grid Arrays�.
Evaluation of residues� resistivity in a specific location on the board. (As compared to �Solvent Extract� evaluation.)
We are: SMT assembly, using standard �water soluble� process with micro BGAs (example: Configuration: 6x8-2, pitch: .75mm (0.029�), size: 7.28 x 6.96mm (0.287� x 0.274�), ball size: 0.35mm (0.014�). Here is my dilemma�
I am familiar with 2 methods of PCB assembly cleanliness evaluation. � Ion Chromatography, (per IPC-TM-650, method 2.3.28) � Solvent Extraction, �ROSE� test (Resistivity of Solvent Extract) (�Zero Ion� or �Omega Meter�) (per IPC-TM-650, method 2.3.25)
With both of these methods the entire specimen (panel, board) in immersed (washed) in �extracting solvent�, only an average board contamination (not area specific) is obtained.
Both methods assume a good access of the �extracting solvent� to the entire board surface. (Ion Chromatography test process attempts more aggressively to gain that access.)
Two questions arise�
What if, in the case of �BGA (or some other � laying very close to the board � CSP (Chip Scale Package)) where access for the liquid is very restricted, the actual washing � part of our manufacturing process � might be limited?
So, in these areas, where the wash quality is problematic, the evaluation of that wash is also problematic.
We can go around this by condoning to destruction some of the �test boards�, by prying (forcibly lifting, creating a gap) suspected components from the board. This breaks the board and / or the component, but opens the access (for the �extracting solvent�) to the restricted area.
The second question is� Since these �BGAs (or other � laying very close to the board � CSPs) are rather small, the quantity of (possibly) entrapped residues under them is also small. When averaged over the whole area of the board (both of the existing methods do that), the results would be quite (probably falsely) acceptable.
Entrapped residues from soldering operation are very corrosives when left on the board. To evaluate if they are there, we will lift components from the boards, (create access for evaluation), but we also need a method to test for residues in a specific spot, not just an average over the entire board�s surface.
Any comments about my concerns?
(I have looked for residues (with microscope 30x, after removing the BGA), visually found none. But I am looking for something more objective and precise)
Thanks for your time and (I hope) comments.
Richard
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