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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


BGA PCB Pad size

Grant Petty

#23056

BGA PCB Pad size | 17 January, 2003

Hi,

We have a BGA that uses .4mm diameter PCB pads on the BGA itself, however we have decided to use .5mm pads on the PCB, to help paste release from the stencil.

Is this a bad thing, having different size pads on the PCB compared to the BGA itself. I guess the solder balls will be slightly irregular in shape, however I wonder if anyone has any experience if this will be a problem or not.

Regards,

Grant Petty Blackmagic Design www.blackmagic-design.com

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#23060

BGA PCB Pad size | 17 January, 2003

How do 0.5 mm pads on the board help paste release from the stencil? Paste release from the stencil is a function of: * Paste * Printer operation * Stencil design

You have no obligation to made your stencil apertures the same size as the pads on your board. You should deign your stencils with the area and apect ratios described in the fine SMTnet Archives to get good release.

You should make the pads on your board according to suggestions of your component fabricator. We don't like it when our designers make pads on the board that are different from the pads on the BGA interposer. We make them sit in the corner and only let them out of the corner after they promise to buy the first round at Jimbo's.

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Grant Petty

#23067

BGA PCB Pad size | 17 January, 2003

Hi,

Our paste release problems have been solved, and we worked on the stencil, and things are working well now. However this board was designed before we knew what was going on, and we thought a little more size on the pads would be a big help.

Is there a specific reason why having a slightly bigger BGA pad than on the package is a bad thing? Is this serious?

We can move it back on the next batch of PCB's however would like to run for a month or so with this design due to time to market reasons. We need this new design because we have taken off a part that's hard to get, and this new design will help us meet demand if every-thing's ok.

Just a bit nervous on the pad size difference! Now I am really freaking out!

Regards,

Grant Blackmagic Design

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#23070

BGA PCB Pad size | 19 January, 2003

You shouldn't freak.

You want the pads on both sides of the BGA ball the same so that the torgue from changes in dimension due to different CTE [ie, interposer, solder, bare board, etc.] is balanced on the top and the bottom of the ball. So, the magnitude of the problem depend on the temperature variation on the product end-use environment.

Put you mind to ease by testing according to SM-786, Guidelines For Accelerated Reliability Testing Of Surface Mount Solder Attachments.

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Grant Petty

#23167

BGA PCB Pad size | 26 January, 2003

Hi,

Ok, thanks for the info, and I had not considered that factor. Is this an issue related to the reflow temperature change in the oven, or a temperature change in normal product use issue?

I will check out the standard, and do you know where it's available from?

Regards,

Grant Petty Blackmagic Design

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#23169

BGA PCB Pad size | 27 January, 2003

Sorry. I mistyped information on the guidelines. It should be: SM-785 - Guidelines For Accelerated Reliability Testing Of SM Solder Attachments

Order SM-785 from: IPC 2215 Sanders Rd Northbrook, IL 60062-6135 847-509-9700

When you are ordering this document, consider also ordering: * PWB-CRT-SG01 - PWB Designer Certification Study Guide * PWBADV-SG02 - PWB Advanced Designer Certification Study Guide

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Grant Petty

#23199

BGA PCB Pad size | 30 January, 2003

Hi,

Great, thanks for the info!

Regards,

Grant Petty Blackmagic Design www.blackmagic-design.com

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