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Reducing Warp after reflow

Reducing Warp after reflow

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We produce a two layer PCB which is on a .062" board ma... - Oct 13, 2022 by PRoy

Do you have the ability to add copper thieving or otherwise ... - Oct 13, 2022 by ProcEng1

No we can't change any of the copper, and that I believe is ... - Oct 13, 2022 by PRoy

I also believe it is the cause of the issue. I guess you co... - Oct 13, 2022 by ProcEng1

If you are running resonable volumes of the board, redesign ... - Oct 13, 2022 by sarason

What about warpage before reflow? Often the boards with unba... - Oct 19, 2022 by Andrey_F

If any changes in design is not possible and you have to sol... - Oct 19, 2022 by Andrey_F

Low Temp solder and a cooler profile should certainly help. ... - Oct 19, 2022 by ProcEng1

Thank you all a but the easy fix - of correcting copper - is... - Oct 19, 2022 by PRoy

I'm sorry to say, but it sounds like your bosses need a real... - Oct 19, 2022 by ProcEng1

I can understand the situation, when there is now possibilit... - Oct 19, 2022 by Andrey_F

Just because you can't add copper to the board, doesn't mean... - Oct 31, 2022 by Evtimov

PRoy

#88446

Reducing Warp after reflow | 13 October, 2022

We produce a two layer PCB which is on a .062" board material. Unfortunately there is a copper inbalance and the boards warp in reflow. Does anyone have any suggestions to reduce the warp? I may try force/restraining them following reflow / cool down and see if that makes any difference

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ProcEng1

#88447

Reducing Warp after reflow | 13 October, 2022

Do you have the ability to add copper thieving or otherwise balance the design?

We had a customer making LED boards with a terible inbalance. Some of the boards would "smile" so badly in the oven that the leading edge touched the top. They switched to all aluminum boards.

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PRoy

#88449

Reducing Warp after reflow | 13 October, 2022

No we can't change any of the copper, and that I believe is likley the primary cause of the warp

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ProcEng1

#88450

Reducing Warp after reflow | 13 October, 2022

I also believe it is the cause of the issue. I guess you could make a fixture that clamps the board flat. Then you'd have to change the reflow recipe. How was the recipe created? Did you actually profile a populated board or just use a recipe that worked? Maybe you could still achieve reflow with a cooler profile. Do you "ramp - soak - peak" or "ramp to peak"?

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sarason

#88451

Reducing Warp after reflow | 13 October, 2022

If you are running resonable volumes of the board, redesign it with equal copper on both sides. One way of achieving this is to just run copper flood planes on both sides of the board, this is a fairly common technique in RF and noise sensitive analog design.

sarason

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Andrey_F

#88466

Reducing Warp after reflow | 19 October, 2022

What about warpage before reflow? Often the boards with unbalanced copper can be warped initially, and you can reject it...

If it's not possible to change the copper, then may be (at least for future lots) it's possible to switch to laminate with higher Tg? Up to TUC family... It's expensive, but this is a payback for poor design. Also for future, if the boards are in panel, check the panel design, is it V-score or milling, may be its possible to rotate half of the boards on 180 deg, to make a panel more symmetrical etc...

But as I understand now you don't have other ways and have to make this boards.

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Andrey_F

#88467

Reducing Warp after reflow | 19 October, 2022

If any changes in design is not possible and you have to solder it, it's a real headache, I know...

I see only 3 options here that you can try consiquently: 1) Accurately profile the board and optimize a profile (go to lower limit of process window for ramp up in preheat and before peak, for peak temp and minimize TAL);

2) Switch to low melting alloy in solder paste (SnBi for example)

3) And if methods above do not make it acceptable (most likely it will not solve a problem completely, but can significantly decrease warpage, to acceptable level) the only way is to use a fixture...

Also you can try to fix board after reflow in flat position and heat it up a bit higher the glass transition temperature (depends on laminate, for HiTg it's normally 160-170 deg) and keep it for an hour or more. But it can have only timely effect for the part of the boards (depends on laminate again and design).

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ProcEng1

#88470

Reducing Warp after reflow | 19 October, 2022

Low Temp solder and a cooler profile should certainly help. I do believe that the unbalanced copper is the culprit, but you could try to prebake some boards. To eliminate some kind of manufacturing defect from the equation. Though I'm pretty sure if that were the case, you'd be seeing delamination in reflow.

Basically, properly redesigning the boards is the only fix. Everything else is a bandaid.

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PRoy

#88471

Reducing Warp after reflow | 19 October, 2022

Thank you all a but the easy fix - of correcting copper - is not an option -actually no changes to the board can be done simply they way it is.... I will look at post reflow bake above TG but I don't want to add a process.... Will look at profile also resistant to change from what works well otherwise.

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ProcEng1

#88473

Reducing Warp after reflow | 19 October, 2022

I'm sorry to say, but it sounds like your bosses need a reality check. You are having a problem with the product, and it's due to bad design. Yet you can't fix the design, and can't add any time to the process to attempt to make it better. There is no magic wand. We all perform mechanical/ chemical processes. They can't reject reality and substitute their own. You also have a duty to whoever buys your products to make them correctly, according to standards.

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Andrey_F

#88475

Reducing Warp after reflow | 19 October, 2022

I can understand the situation, when there is now possibilitie to change board design, or something like this at the moment, and now you have to work with what you have now... Customers and management can bring a stupid situations...

But I'm agree with ProcEng1 - why not to try with profile and low melting alloy? There is no problem to modify process, or switch to another solder paste, especially if you have a good profiler with a good software (KIC for example and others)? It is easier, faster and more effective to decrease warpage initially, then try to fix it after reflow. In this case you have to make a fixture, find an equipment, spend a lot of time for heating of each board/panel etc. And effectiveness is a question - it can be only for some time, that's why I mention about this outside the main list...

But it's up to you, of course, and please let us know the result and method in any case!)))

Thx!

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Evtimov

#88547

Reducing Warp after reflow | 31 October, 2022

Just because you can't add copper to the board, doesn't mean you can't add copper to the panel. Redesign panel, where board house can add copper to the panel to balance the board a little better. I would advise against low melting solder alloys on current design - if boards are warping so bad, these Bismuth alloy joints will probably crack right in reflow cooling section.

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