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Step down stencils using CBGA and 20 mil QFP's

Ron Costa


Step down stencils using CBGA and 20 mil QFP's | 21 October, 1999

We are a contract manufacturer currently building a job with IBM CBGA-255 devices and 20mil QFP's that are close to each other. We use a 6 mil laser cut and electro polished stencil with one to one apertures on the BGA's. The customer is experiencing many opens in the field on the BGA devices. IBM recommends 8-10 mil paste thickness for the BGA's but, I have large 20 mil devices within .500 of the BGA's. We are thinking about using a step down stencil from 9 mil to 6 mil. Does anyone have experience using step down stencils on this type of application? Can anyone offer any assembly process suggestions? All feedback appreciated.

Specs: Semi -automatic screen printer .063 pcb electroless nickel/immersion gold plating 63/37 OA, type 3, 90% metal paste Metal squeegee blades

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Re: Step down stencils using CBGA and 20 mil QFP's | 22 October, 1999

Ron, if I am reading this correctly the 20mil parts are not having problems on a 6mil stencil. If you increase the entire thickness of the stencil to 8mils, there should not be any issues with the 20mil components. The supports you are using could possibly be moved to support more under one side of the board allowing more paste to be applied to the sloping side. This might possibly work with adjustments to the squeegie down pressure. Or adjust the board loading orientation so that the squeegie blade crosses the 20mil components first then attacks the BGA. You can change squeegie speed from fast to normal at a certain point. The paste will be less on the fine pitch while maintaining normal heights on the BGA. Then you can increase the stencil thickness without paste height problems. I hope this helps...good luck.

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Re: Step down stencils using CBGA and 20 mil QFP's | 25 October, 1999

Ron: You're really talking about a step-up stencil in the area of the BGA. I don't think your problem is a lack of solder, but who know?

I think you should do some sections on returned boards from your customer to help determine the source of the problem.


Dave F

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