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Throiugh Hole pad missing after selective soldering

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#87828

Through Hole pad missing after selective soldering | 13 April, 2022

Hello! We have a strange effect. Some pads of big trough-hole DDR slot just dessapear. Initialy it was soldered with selective soldering process (leaded, 290 degrees, ImSn finish, very short storage). Then some connectors with poor barrel fill we have try to rework with the same selective solderig with addional flux and a bit higher temperature. Afte that at looks like pads was washed away complitly, or dramaticaly decreas in size down to size of through hole. Is it possible that due to overheat, poor pcb quality or by some other reason soldering pad was complitely detached from throuh hole and blow away with a solder? This is quite big multilayer motherboard, that's why we try to make a rework with selective soldering machine, not with soldering station. Boards was prechecked, no issues with lifted lends etc...

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#87832

Throiugh Hole pad missing after selective soldering | 13 April, 2022

If it was ROHS, I would say that the pads are definitely dissolved away. And it sounds like that is the case here.

Your best bet is if it is class 2 and the barrels are at least 50% then push for them to be accepted.

If you are trying for more barrel fill from the bottom then you have to melt the solder that is in the barrel first. And if you had trouble getting barrel fill in the first place, this makes it much much more difficult.

Avoid having insufficient barrel fill in the fist place. Make sure thermal relief techniques are used in the PCB.

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#87834

Throiugh Hole pad missing after selective soldering | 14 April, 2022

Thank you for reply, Stephen. No, As I wrote it was leaded process. I heard that it is possible with lead free solder at higher temps an dwell time. At NPL Defect Database I've find this case wich looks exactly the same: http://defectsdatabase.npl.co.uk/defectsdb/defect_details.php?d_pkey=145 but it talks about copper corrosion at Lead-Free process. It was first modules and we adjust the process. Despite that we already successfuly produce such complex boards (it's a server motherboard) at first 2 boards some pins have very insufficient barrel fill, and operator try to repair it by increasing temp to 310 deg and dwell time 6 sec. Usually it helps, but now for the first time I've seen copper corrosion at leaded process...

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#87835

Throiugh Hole pad missing after selective soldering | 14 April, 2022

Also at few boards we see excessive voiding, extremily excessive. During solderind some bubbling can be observed near the soldering point. Boards was dried before assembly. May be we have a problem with too thin metallisation - that's why poor adhesion and too big outgassing? Just analysing all this problems whith this butch I keep in mind that we have a problems with PCB's But we have not too much of them, and they are quite expensive, not too much that we can do to check it...

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#87837

Throiugh Hole pad missing after selective soldering | 14 April, 2022

If it was ROHS I would suspect copper migration. In my experience an acceptable copper finish should last up to at least 6 reflow cycles. If you have bubbles or blow holes in the solder joint, that is a sign of voids in the walls of the copper barrel. I would consider these boards defective and not reliable. Ask your supplier to remake them with thicker copper.

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#87842

Throiugh Hole pad missing after selective soldering | 15 April, 2022

Thank's Dennis. I'm also now think in this direction. Process was adjusted and soldering are good. But one from five boards can have this effect with bubbles and strange uneven distribution of solder inside the barrel. Probably copper is too thin and/or initially have voids and cracks. Also probably couse of weak copper finish pads was removed after rework cycle...

I have stop the production and wait for result of microsection of bare boards and assembled one

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#87851

Throiugh Hole pad missing after selective soldering | 18 April, 2022

As far as hole fill, as was stated above, class 2 only requires 50% up-fill. In order to get better up-fill on the initial pass, you may need to consider more pre-heat. Also, better application of flux. Are you using a drop jet, or a spray? You might not be getting the flux up the hole. I have used top-side application of flux, but only when I could fully access the top to clean afterwards.

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#87852

Throiugh Hole pad missing after selective soldering | 18 April, 2022

Thanks for all comments. Process was adjusted normally, problem was in PCBs. Microsection of bare boards confirm that the copper thickness in troughholes was unsuffient, sometimes below 10 microns, on soldered boards where there were a problems during soldering it shows a voids in a copper, cracks between pads and through hole copper... So the problem was in boards.

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