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SMT electronics assembly manufacturing forum.


To use a mini stencil, or not to use a mini stencil.

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#48738

To use a mini stencil, or not to use a mini stencil. | 29 March, 2007

Who out there uses a mini stencil to paste some paste when they replace a BGA and who uses just flux?

What are the benefits or both and more importantly - how many times can you do both on the same component / board?

Thanks in advance! Chrissie

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#48753

To use a mini stencil, or not to use a mini stencil. | 29 March, 2007

We always apply solder paste to either the PCB pads or the BGA balls. I have found that only using liquid or tacky flux does not always ensure proper wetting of the pads. Also The BGA sits closer to the PCB making cleaning and endoscopic inspection more difficult. Appying solder paste facilitates the wetting and breaks the surface tension of the solder ball allowing the solder ball and the paste to completely reflow together and create a good solder joint. I have seen many times BGAs placed with just flux where the BGA balls reflow and colapse but do not form a joint with the pad. Applying solder paste to the PCB pads can be an art... thats why we apply paste to the BGA balls 95% of the time. Appying paste to the balls also lets us use paste when there is very little or no real estate around the BGA site.

I have read in several component data shhets that recomend between 3-5 heat cycles as the maximum. So that means 1 to place the BGA, 1 to remove it, 1 to reball it, and one to place it back again. Thats four heat cycles for one rework cycle. I know people do more all the time with success. I think the key is to make sure the BGA is "dry" before any rework is attempted by baking the BGA according to the coponent data sheets recomended baking procedure.

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JD

#48756

To use a mini stencil, or not to use a mini stencil. | 29 March, 2007

Ewwww metal mini stencils? Try http://www.circuitmedic.com/products/stencils.shtml We use their flextac stencils all the time and they work perfect. You can get a couple uses out of each of them if youre careful too. Just line it up under a scope and set the adhesive and youre ready to go. No affialiation with Circuit Tech Center, just a user.... Using flux only is risky especially if using a fpga or something prone to warping. The solder paste if nothing else gives you a margin of error for that.

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#48772

To use a mini stencil, or not to use a mini stencil. | 30 March, 2007

Hi Chris,

Mini stencils are a pain to line up and then holding it still while trying to print is almost impossible. We used to use tacky flux but found the Stencil Quik gizzmo from Best works the best. Give it a try.

http://www.solder.net/stencilquik/default.asp

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JD

#48782

To use a mini stencil, or not to use a mini stencil. | 30 March, 2007

We have had issues with using the ones from Best. The holes are not always cut correctly and the edges (inside the holes) are not well defined because of the kapton material they use maybe. Plus you leave the mini stencil on the PCB under the BGA which could cause problems potentially with some customers. I have tried many different package types of the StenQuik and Flextac and we found the Flextac to much better. Get samples of both and check em out for yourself...

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