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Connector Bowing During Reflow Process



Connector Bowing During Reflow Process
I have a connector that is bowing during the lead-free soldering process. When the connector is put through an oven on its own no bowing takes place. What is causing the connector to bow? Phil Zarrow and Jim Hall, The Assembly Brothers, address this question.
Board Talk

Transcript


Phil

And welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers Pick and Place, who by day go as ITM Consulting. You may recognize our voices. We are here at Board Talk to talk to you about process, questions, problems, situations.

Today, Jim, we have a soldering problem. This is from B.S. I have a connector that is bowing during the lead-free soldering process. When the connector is put through an oven on its own no bowing takes place. What is causing the connector to bow?

Jim

Non-uniform heating. What would cause something to bow? Heating. The obvious issue to me is non-uniformity. When you put it on the oven, on its own, all sides of the connector body are exposed to heat so it heats up relatively uniformly. It doesn’t bow.

They don’t talk about whether they are wave soldering or reflow soldering. But in either case, the side of the body of the component that is closest to the PCB is not going to be heated as easily. My suggestion would be to go back to your soldering process and adjust your preheat in wave or the preheat in your reflow, whatever it is.

You may want to put a thermocouple on the bottom of that component. Make sure that it is getting up to the proper temperature. You may want to put one on the top and the bottom and check it during preheat. Make sure that the top isn’t heating really fast and expanding and causing it to warp. To me, it seems pretty straight-forward that heat that makes it warp is non-uniformity.

The classic example, think about a reflow oven where you are getting convection heating from the top and bottom. You put a large body connector down on the board that the access to heating is around the leads, either through-hole or surface mount pads on the bottom, is restricted. So, you probably have to slow down your preheat, maybe add a soak to your profile.There are different strategies. The key is to put a thermocouple on there and make sure the top and the bottom of the connector are heating relatively uniformly.

Phil

And B.S. says that this is happening in an oven so it is during the reflow process.

Jim

When the connector is put through an oven on its own no bowing takes place. Yeah, I guess it applies to reflow.

Phil

Right. But what you are saying is if this was going through a wave soldering machine and the attributes that Jim is talking about would apply to a wave solder machine as well.

Jim

In wave soldering, you also have different kind of pre-heats, like calrods or quartz lamps which might be adding to the non-uniformity heating.

Phil

Right. And whether you are heating from above, below, or both, whatever. So good wisdom Jim, good universal wisdom. Imparted all around there, good universal wisdom. Hopefully that answers B.S.’s question and situation there. We hope that you aren’t involved in any bowing but if you are whatever you do, make sure you’re not soldering like my brother.

Jim

And make sure you’re not soldering like my brother.



Comments

I would guess as the board and the connector go through the oven (same thing for wave soldering) the board and connector expand at different rates and different amounts, CTE mismatch. You can also get some bow in the board.

So when it comes out of the oven and the solder cools, the connector is locked in place by the solder. Board contracts, connector warps. Even worse if you have the board bowing.

So as the brothers discuss, you may have to adjust your oven profile. If you get a lot of board bow, you may want to fixture the board also.
Alan Woodford, NeoTech
Is it possible your PCB is guilty, and the connector is just the scapegoat? I've seen a PCBA warp, and then relax as it cools. If there were a long edge connector, and the PCB relaxed only after the connector leads were soldered, it might introduce enough flexing stress to bend the connector. /perry mason.
Alan Couchman, Process Sciences, Inc.

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