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BGA rework stencils ( down to 0.65mm pitch)

MarcS

#21923

BGA rework stencils ( down to 0.65mm pitch) | 14 October, 2002

Hi, Has anyone had expereince in the design or use of mini rework srtencils for uBGA applications? What was the deisgn guide lines used. I am looking to rework 0.7 & 0.65 mm uBGA parts with standard hot air, without split prism placement guides. It is on STD 1.6mm FR4 with around 2mm clearence for devices. Part perimeter visable on board. I have tried conventinal techniques of applying flux to lands, but I cannot inspect after rework as the height off the board is not enough for my optical inpection machine. I am not confident with the consistancy / reliability of this method, even though a good profile inside the rework nozzle has been established with thermocouples.

Your help / expeirence would be appreciated

Regards

Marc

Regards

Marc

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RDR

#21926

BGA rework stencils ( down to 0.65mm pitch) | 14 October, 2002

First off, are you planning on placing these parts with your placement machine since you mentioned that you do not have an alignment system for rework? I would be leery of handplacing a BGA into paste by steady hand and craned neck alignment. Second, When you state that you are not confident in the flux on pad only method I am curious as to why not? You are not (in my opinion) going to be really be changing anything by applying paste and then reflowing exept the possibility of varying paste volumes. Have you been experiencing failures?

Anyway, I have used the microstencils and have pretty much gone away from them for the following reasons.

1. unless you have some type of holder/positioner (which is available)it is difficult to apply paste that is both registered and of consistant volume.

2. Edge clearances sometimes need to be in excess of 2.5 mm.

Finally, I am not saying the ministencils don't work because they do, but I have found that they seem to add more variables and cost unecessarily

And thats my take on the subject

Russ

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MarcS

#21941

BGA rework stencils ( down to 0.65mm pitch) | 14 October, 2002

Russ, Thanks for the feedback.

Yeh, we are placing them in production using a full ball detect Yamaha placement machine, 5 thou square stencil. etc.etc.

The only reason I raised this concern was from a rework point of veiw (actually, the board design had the wrong pin out on the board, so we trying to find out why, hence we performed lots of rework). Using flux only does not allow enough clearence of the board to enable inspection aferwards ( unless you have x-ray, which I don't). My concern is quality of reflow- I have already verified the profile via thermocouples. I guess I want to be sure my placement and ball grain structure is bang on, after rework. Back to mini stencils, can you please provide me more of what you did to get the process working for a design point of veiw.

Regards

Marc

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RDR

#21942

BGA rework stencils ( down to 0.65mm pitch) | 14 October, 2002

When I used the micro-stencils, We purchased the holder from the stencil manufacturer. As far as design we used 5mil stencils for uBGAs,6-7 mil for larger 63/37 balls, and 8 mil for CBGAs. Aperture openings used were slightly smaller than what we would use for production on 50mil pitch, while on uBGAs we used the exact same aperures as our PCBA stencil (this was done to aid in the release). I would contact mini micro stencil inc.(I AM NOT A REP!) for details on your application. They seem to be pretty up to date and can probably help you more with specifics. If you do decide to use the mini stencil approach make absolutely sure that every aperture has released or you will definetely get opens! since the additional height from the paste will keep that ball hangin in the air and those little babies don't collapse enough to enable contact between ball and pad (I know this from experience).

Have you thought about trying a larger ball diameter to aid in inspection?

Hope this helps

Russ

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