Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.




CBGA ON BOTTOM SIDE | 26 April, 2003

I'm working on a protobuild for a pcba. THis pcba uses double-reflow, paste-in-hole process. On bottom side there is a Ceramic BGA, size 35 X 35 mm, thickness 6mm, over 900 balls count (ball pitch 1mm)and it weighs a hefty 20 grams. Printing uses 6 mils stencil with 29 mils round opening (larger than pad size of 27 mils. When processing the top side, will the CBGA (on bot side)drips or even fall-off from oven ? Any formula to calculate if the surface tension of the solderballs is enough to overcome the weight of the CBGA ? Even if the CBGA stays intact on the bd, any potential long-term reliability issue for such CBGA that undergo such process ??

Anyone with any inputs & advice will be greatly appreciated !!


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CBGA ON BOTTOM SIDE | 28 April, 2003

Double sided weight of component to total pad mating area ratio: 30 gm per sq inch (50 mgm per mm2)

Go to the sites of these consultant-people for additional background [I'm not sure exactly where on these sites to look, but both are laden with good SMT knowledge] * Phil Zarrow * Bob Willis

Collecting masses and areas for common components is a pain in the neck. Get free data [that might work (It's at least a good starting point for you.)] from "Regulating Table Motion Effects On Component Placement", Table 1, 1/03, EP&P magazine, [Unfortunately the table is a picture. So, you will not be able to paste it into Excel. But hey!!!]

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CBGA ON BOTTOM SIDE | 30 April, 2003

Lwei, Our boards are double sided and have BGA's on one side. We always run this side first so we can x-ray inspect the BGA's for short circuits, then run the second side. Depending on what industry you are in you may have some problems.. we work in aerospace and mainly run by European Space Agency (ESA) rules. By running the BGA's on the first side we admit to "re-working" (as opposed to "repairing") the BGA's. This can cause problems down the line with other resitriction such as only three "reworks" on one board etc if you need to rework anything else on that board. We have had no problems with movement of BGA's, them falling off, or the integrity of the solder joints, and we've been at this for over two years now. Hope some of this is of help, Chrissie

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Chris Lampron




This is not a permanent fix for the problem. We have had similar experiences with double sided boards. As a fix, we have applied water soluble solder mask to the edges of the component. This holds the component adequately for second side processing. The material is removed at the wash.

This will work in a pinch, but is not a resolution to the problem.

Good Luck


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