Hello technet members,
My company sub-contract all our PCB assemblies, so we rely almost
exclusively on the sub contractor design and process to make good soldering
joints and PWB.
We have been using micro-BGA at 0.8mm pitch extensively for the past 2
years. We never had 100% yield on our PCBA , more like 96%. Some of our PCB
have 3 uBGA so this translate into even worse yields. 80%of the failed
component had opens under the BGA (80%).
PWB ISSUES
We have identified one major source of problem: Unstable PWB.
The typical board size are 11"L X 2.5"H and 11" X 11", 8 or 10 layers,
0.062" thick, Top + Bottom components. HASL finish.
The boards showed excessive warping , bow, twist sometimes in combination
.There is no trend in the direction of the warping, it is more like
randomized. After each reflow cycle, the situation worsen's. I have asked
our PCB designers if they have balanced the copper and it is balanced. I had
a hint that our bare board manufacturer is pressing the boards before
delivery but it was never confimed by them. It is curious to see that our
inspectors never saw a warped board from our PWB supplier. My though on
that is that if it happens during reflow then it should have happened during
the HASL plating.
Note: the profile of the board with the thermocouple on the PWB at the
assembly supplier has been validated.
QUESTION 1 : Can i assume from the above that our PWB supplier is delivering
unstable boards ?
QUESTION 2 : How could i check the bare board stability? Is there a
procedure / standard that exists and can be specified in the requirements?
Any suggestions.
QUESTION 3 : If i go with a high Tg material > 175C , would that minimize
the warping. For now i connot change supplier
QUESTION 4 : Are the assembly yields of ENIG better than HASL ?
PCB ASSEMBLY ISSUES
I started looking at the stencils used by the assembly house. In the
process of doing that, i realized that there was one square and two circular
aperture of different sizes for the same pad size and component on three
different pcb. There is no standard practice for the design of the
stencils.... YARK!
Now i want to determine the best stencils parameters to use. help me !
Stencil laser cut 0.005" thick,
uBGA of 0.031" pitch and nominal ball size of 0.020" ; 144balls.
The landing pad size is 0.014"
The solder paste is alphametal OA
The current aperture is 0.014" circular
QUESTION 5 : Is this a good stencil aperture , should we overprint?
QUESTION 6 : What would the technet users recommend to improve the stencil
design ? Size, shape, specifics.
QUESTION 7 : What should i expect in term of assembly yields per solder
joints for 0.031" pitch uBGA for a bad, good, excellent process.
I looks like there are different schools of thought, one is circular and the
other one square with beveled edges.
I thank you in advance of any suggestions and recommendations.
Regards
Michel Arbour
Quality Engineer
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