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Rigid Flex Wave Soldering

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#48871

Rigid Flex Wave Soldering | 4 April, 2007

Anyone have experience wave soldering rigid flex assemblies?

I've just picked up a new job from a customer; it's a rebuild for the company, but new to me. The customer had previously taken their business elsewhere due to delamination problems with our process; but now they're bringing it back to us.

I've spoken to the manufacturer, and it looks like the solution to my delamination problems is pre-baking the assembly before wave soldering. For the batch I just produced, this seemed to work out fine...no evidence of delamination on the flex.

However, I seem to be having an awful lot of blow-holes and pin holes in the solder joints. I'm looking for some guidance as to how to minimize (preferable eliminate) the blow-holes and pin-holes. The rework effort is painfully time-consuming.

The pin-holes and blow-holes that I'm seeing are remeniscent of lead-free solder issues. However, these units are being produced with regular leaded solder. We manually flux the boards (spray bottle) since we don't have a spray or foam fluxer set up on our wave machine.

Any recommendations for processing these units to reduce the pin-holes/blow-holes?

Thanks, ..rob

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#48872

Rigid Flex Wave Soldering | 4 April, 2007

Maybe more baking time is needed.

Topside board temp too low.

Moisture trapped in board.

Top of through hole capped by component and trapping flux.

http://www.speedlinetech.com/electrovert/electrovert-troubleshooting/index.htm

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#48873

Rigid Flex Wave Soldering | 4 April, 2007

Rob: How do you rework blow-holes and pin-holes?

PTH plating thickness SB 1 thou or greater. Epoxy is hydroscopic. So, you should control the environment and/or bake the moisture from the boards before soldering. Search the fine SMTnet Archives for baking recipes.

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#48876

Rigid Flex Wave Soldering | 4 April, 2007

Pete,

Top-side temp might be an issue. What's the easiest way to check it? I seem to recall some thermal decals that we used to use a couple of jobs ago.

I spoke with the flex manufacturer, and they recommended 4 hours at 250 degrees pre-process baking. We did that. I suppose that there is still moisture trapped, but would hope that the baking would take care of it.

The components being soldered are connectors, and they do sit pretty tightly to the board...so, yeah, they could be trapping flux. We hand spray flux on the bottom side, and are using a flux bottle to get flux underneath the components on the topside. Maybe we should try one without fluxing the top side...though, we're concerned about not getting enough top-flow (more-so since we're unable to see if there was top flow).

Thanks for the link! I'll be doing some reading!

..rob

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#48879

Rigid Flex Wave Soldering | 4 April, 2007

Dave,

We're reworking with flux and solder. We're observing "exploding" joints, as well...as though something is trapped in the joints (either excess flux not burning off during solder, or residual moisture).

As mentioned above, I spoke with the flex manufacturer, and they recommended 4 hours at 250 degrees prior to wave soldering.

I did a search in the forum archives earlier for rigid-flex, but didn't find much. Some suggestions about baking, but not much direction on how much/how long.

Thanks, ..rob

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#48885

Rigid Flex Wave Soldering | 5 April, 2007

On finding baking recipes in the fine SMTnet Archive, your problem is not in the flex portion. It's in the rigid [FR4] portion.

On baking advice from experts look here: http://www.circuitnet.com/experts/EQ10120.shtml

Finally, on reworking your outgassing, this is very bad practice, because what your people are probably 'painting' solder over the openings of your little volcanos in the solder connections. This creats a void in the solder connection.

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#48888

Rigid Flex Wave Soldering | 5 April, 2007

Dave,

I've seen some instances of solder as you mentioned "painted" over the joint. Almost looks like a seam between the added solder and the originally waved on solder.

Typically, I've seen that in the past from an incomplete solder joint. One where the original joint does not flow, and the new solder melts on to it.

The process my folks are using is to flux the joint, heat it till it flows. Solder is added if/when the original joint flows through the hole too much, and results in insuficient solder on the bottom side of the assembly.

cheers ..rob

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#48891

Rigid Flex Wave Soldering | 5 April, 2007

Please note that pin holes and blow holes are not always defects. The latest IPC 610A Rev D workmanship standard has them as process indicators- depending on Class. I don't have the document handly while writing this. It would be good to look that up to get the specifics.

I would recommend not to flux the top side on those connector sites and see what you get. If the top side is preheated to the correct spec, then you should be getting hole fill. Your goal is to try to get as close to a zero delta between top side and bottom side board temp just prior to hitting the wave.

I agree with Dave on the rework issue.

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