Michel

One potential issue with uBGA is that the density of the holes chops up the
glass bundles in the dielectric, resulting in localized variations in CTEs.
This might be a contributing factor to the warpage.

You shouldn't have to assume anything about the stability of your boards.
If you suspect a problem, I recommend visiting your fabricator during the
next build of your boards with the intention of inspecting the boards after
lamination.  If they are warped then, your suspicions are justified.

> ----------
> From:         Michel Arbour[SMTP:[log in to unmask]]
> Reply To:     [log in to unmask]
> Sent:         Friday, January 12, 2001 11:54 AM
> To:   [log in to unmask]
> Subject:      [TN] uBGA assy yield problem : PWB not stable and stencil
> specification
>
> 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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