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BGA Pad with Irregular Pad size

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#72691

BGA Pad with Irregular Pad size | 5 September, 2014

Hi,

We have a problem with our BGA Pad. Our PWB manufacturer make the BGA Pad in irregular form. some big, some small. Please see the picture on attachment.

BGA Detail BGA Pitch 0.5 mm BGA Ball 0.3 mm BGA Pad (big) 0.2 mm (as per gerber) BGA Pad (small) 0.13 ~ 0.15 mm (not actual measure) Stencil apperture size 0.2 mm (as per gerber size)

My questions are, 1. What is the effect of small BGA Pad on the PCBA? Currently we have high rejection rate for this PCBA, but we cannot find (yet) it's correlation to the BGA. 2. Will there'll be reliability issue due to this? I'm thinking due to the pad - ball connection is little, it might easily crack. Please correct me if I'm wrong. 3. Can I used these boards? 4. If rejected, should we reject the whole lot? The manufacturer change the spec on their own initiative, and without our approval. Because they said there are design constrain (trace between BGA Pads, so they reduce these pad's diameter)

regards

Edri

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#72693

BGA Pad with Irregular Pad size | 5 September, 2014

Hi, Edri,

It's not uncommon for PCB fab houses to change pad sizes like that, due to design constraints....however, it always aggravates me when they do it without checking with me first. In my mind, that's the first sign of potential bad issues with a fab house...if they don't communicate about something like this, I'm always concerned about what other potential issues they may not tell me about.

On to your questions. 1. The potential effect is for an inconsistent joint between the balls and the pad. 2. This is a potential reliability issue...with such small targets, you could end up with poor connections. After looking at the pictures, I'd be concerned about some boards possibly having solder mask over the smaller pads, as it looks like they've gotten particularly small. 3. I can't help you make that determination...I'm not fully versed in BGA's. On the surface, I can tell you that I don't like it! 4. Yes, if rejecting, I'd reject the entire lot, and remake the boards I'd try working with the designer, fab house, and BGA manufacturer to get to a determination of what is allowable, functional, and process controllable. It's possible that you could use these boards with no issues...but, I'd definitely work to get all parties to sign off on that before running the boards.

Cheers, ..rob

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#72702

BGA Pad with Irregular Pad size | 7 September, 2014

Edri

PCBA design and pad geometry is one of the critical factors for BGAs. Typically the pad size for collapsible balls is 80% of the ball diameter. Pad size consistency is critical. In your situation what will happen the larger pads will wick the solder from the part as the smaller pads will hold the part higher off the board. This will cause opens at the part pads.

1. Reject the boards. 2. Never let your board house modify your gerbers without checking with you first. If they insist on doing it find a new board house. FAB houses love to cut corners which makes their lives easier and make your life miserable and expensive.

Sorry to say if you process these boards you are set up for failure. Using properly designed PCBAs makes half the BGA process bullet proof, the other half being placement and reflow. Make your board house and designers do their half.

20 years experience with BGAs.

Good luck

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