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BGA reflow on bottomside

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aj

#52587

BGA reflow on bottomside | 20 November, 2007

Hi all,

I have never had to run a product with BGAs on the bottomside of a double sided board. I was just wondering if this is a done thing ? or should I stay well clear.

any replies appreciated.

aj...

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#52590

BGA reflow on bottomside | 20 November, 2007

Only on the bottom side?

Run that side first .

We usually determine our run process based on the heaviest components, and propensity for falling off during the second reflow process. If there's only the one BGA on that side of the board, we'd tend to run that side first.

We haven't run into the issue yet, but, I'd have to assume you could do it. We don't have a chain rail in our reflow, so everything runs sitting on the belt. In instances where parts are falling off the first side, we've done two things: 1. adjust the reflow profile to attempt to hold the first side below reflow while reflowing the other side (not terribly effective...but I keep trying it anyway. I figure why have top and bottom side zones in an oven if I can't do this!). 2. Run the boards on aluminum foil (more effective, at least in retaining the parts; although this requires re-profiling the other side, as the aluminum foil tends to sink a lot of heat).

You could create a customized reflow fixture that supports the entire bottom side of the board, and prevents components falling off during second reflow.

cheers ..rob

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#52593

BGA reflow on bottomside | 20 November, 2007

It can be done. But as Rob states, use edge rail oven. We have a product 18 inches long by 9 inches wide with BGAs on both sides.

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aj

#52605

BGA reflow on bottomside | 21 November, 2007

Thanks guys.

Chunks - how many zones do you have in your oven ?

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#52607

BGA reflow on bottomside | 21 November, 2007

Rob: You say, "We usually determine our run process based on the heaviest components, and propensity for falling off during the second reflow process. If there's only the one BGA on that side of the board, we'd tend to run that side first."

We agree with the first part, "We usually determine our run process based on the heaviest components, and propensity for falling off during the second reflow process."

We disagree with the second part, which contradicts the first part, "If there's only the one BGA on that side of the board, we'd tend to run that side first."

We usually determine our run process based on the heaviest components, and propensity for falling off during the second reflow process. Period, end of story.

Our old friend Bob Willis theorized that most BGA and flat packs reflow well on the second side, except for the BIG-Honking ones. The BIG-Honking ones fall, not because of too much weight, but because of their size. The amount of flex in BIG-Honking ones is relatively large [because the part is large] and is in a different sequence during heating cycle than the board. [Out of respect for Bob, "BIG-Honking" is our term, not his.]

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