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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


BGA coplanarity

jkost

#2111

BGA coplanarity | 27 December, 2000

As part of an Intel erratum, they changed their BGA coplanarity spec from .006" to .008". The package is 23 X 23 partial array, .050"(1.27) pitch, 1.220"(31) square. We apply paste onto BGA pads with a 6-mil stencil.

From a process perspective, should I be concerned?

John K.

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#2112

Re: BGA coplanarity | 27 December, 2000

From a process perspective, I don�t know if you should be concerned ... You've told us nothing about your process.

THE BOOK

JEDEC MO-51 allows 6 thou warp in smaller [1mm pitch] BGA and 8 thou in larger [ 1.27mm and 1.5mm pitch] BGA. The increase Motorola laid on you was allowed by JEDEC because the manufacturers had a tough time meeting the 6 thou requirement in the larger packages.

PRACTICE

As long as you maintain symmetry of materials and balanced copper, warp should not be an issue. Generally, balanced product has warp so low that it is essentially unmeasureable.

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Terry Burnette

#2113

Re: BGA coplanarity | 3 January, 2001

John, There is one process issue that occurs with the larger BGA packages which have coplanarity near the high end of the .2mm spec.. The defect is commonly referred to as "ball in cup" and "head in pillow". It is an open solder connection whereby the BGA solder sphere sits directly in the solder on the PCB yet does not attach. I have found this type of open solder connection occurs with reflow solder profiles which soak between 150C to 180C for more than 70 seconds. This type of defect occurs when the flux in the solder paste, which is applied to the board, dries prior to the solder melting. Once the flux dries, it creates a harden coating between the PCB solder and the BGA solder sphere. The molten BGA solder sphere and molten PCB solder can not break through the hardened flux and can not form a solder connection. Normally, the open connections occur on the corner BGA solder spheres which have the highest coplanarity. Regardless of the coplanarity of the BGA at room temperature, all Plastic Ball Grid Arrays will have near zero coplanarity once they reach the glass transition temperature of the plastic mold cap. For most PBGA's the glass transition temperature is between 190C and 200C. Even though the PBGA's will flatten at these tempertures many solder paste fluxes will be to dense for the PBGA solder sphere to penetrate, if the reflow preheat has to long of a soak time.

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