BGA rework and inspection

Hi Yannick.

This is an issue that we are just recently getting into but here is some ifo that I have gathered so far. I have seen/ used 2 different rework machines. The one we have bought is a 'PACE TF2000' which was at the lower cost end of the market, but still has the necessary vision alignment and profiling capabilities and is capable of doing a pretty good job. If you have a lot more money to spend then I have seen the 'Conceptronics Freedom 2000' in action and it is very good, especially when developing new profiles.

X-ray capability is a must but I cannot advise which machine is best. Our x-ray machine is located in one of the deepest darkest corners of the plant and I don't know the make. I have however seen it in action against a much (much) cheaper machine. The cheap machine which was old and is probably no longer available, showed the joints well enough to establish the shape of the balls and whether there was any shorts, but it did not show up voids within the balls. these only became apparent when we put the board on our own more expensive machine. just something else to consider... As an additional inspection tool, we are now considering buying an Ersascope which looks under the package and allows you to see the balls themselves to check for cold solder, proper fillet etc. Details of this kit is available at http://www.ersa.com

Again, being new to the process, you might find (as we did) that you can remove a BGA without any problems, but on pad cleanup, you get break down of the solder resist between the pads and vias (I can't comment on your boards.. I'm just bringing up problems that we have seen). This leads to bridging, and the vias rob solder from the joint. We do not believe we will be able to 100% eliminate this issue, and some degree of resist touch-up may be required on the occasional board, but we have made it a darn sight better by using a 'PACE VF100' rework unit, which both blows hot air and has vacuum to 'hoover' the pads clean. This limits the mechanical stress you introduce by wicking the pads with braid.

Another thing to think about might be the way you re-print the boards (if indeed you are intending to do this), prior to fitting the replacement part. Right now we use micro stencils cut from larger screen printing stencils and we struggle to obtain a good gasket when printing an individual BGA on a populated board. This might improve with practice, but i have just found another possible solution from a company called Circuit Technology. They have laser cut polymer micro stencils for BGA rework that are self-adhesive and apparently leave no residue. This solves the gasketing problem. Another company I know has tried one and says it was re-useable up to 10 times before the tac wore off.You can obtain a free sample in just about any size from their website: http://www.circuittechctr.com

One or two other points I picked up from a company who specialise in BGA rework are : 1) It is almost always better to flux a component before removal as it maked pad cleanup much easier. 2) Clean the pads immediately the BGA has been removed as this utilises the residual heat in the board and only the lightest mechanical contact is needed on the pads. 3)Rosin based fluxes are better for the rework process than aqueous or no-clean flux.

I hope this helps in some way.

Dave.

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