Cookery

Toby Seckshund

In the chip specs it is stated that the soldering temp should be ramped up to a staging point, then ramped up to the melt point and the ramped down within a contolled time.

Then I read stories of people toasting the PCB, or using heavy skillits to melt things down. Neither of these methods could be shaped to the specs that precisley so just how brutal can you be before the cct is truely "toasted" (beyond all redemption)

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Bill Henning

No idea... yet

FYI, I plan to use a heavy duty SSR, an IR thermometer, and a propeller to do ramping profiles later.

Toby Seckshund said...
In the chip specs it is stated that the soldering temp should be ramped up to a staging point, then ramped up to the melt point and the ramped down within a contolled time.

Then I read stories of people toasting the PCB, or using heavy skillits to melt things down. Neither of these methods could be shaped to the specs that precisley so just how brutal can you be before the cct is truely "toasted" (beyond all redemption)

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James Long

Toby,

The ramp profile of reflow is geared more towards heating too fast, cooling to fast and getting solder to perform well.

The Heating of a board is restricted to 3 degree of Celsius per second. This is a standard set by the IPC.

The ramp profile is set by the solder manufacturer to get the solder to perform for the best results.

The cooling is also set by the solder manufacturer.

The most important thing with regard to chips, is the amount of time above the reflow temperature. At that temperature, chips will start to melt or start to degrade on the inside.

The profile itself, is recommended by the solder manufacturers. With leaded solder you can deviate a great deal, and still get acceptable results. Lead-free on the other hand is not very forgiving. If you heat too slow, the flux will flash off before the solder becomes liquidus. If you heat too fast, the solder will not have a sufficient time to activate, and will result in poor solder joints.

So, the short answer to your question: If you are using leaded solder, you can "toast" or "skillet" boards and get acceptable results at home. In a production situation, with many boards, you may not get so lucky.

I know that is probably not the answer you were looking for, but it really is a complicated question.

James L

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James L
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Toby Seckshund

I have seen the usual videos of this work but I have always wondered just how the heating surface's thermal delay, and then the component layer's thermal delay behind that could be reconciled to the dire warnings dished out with the chip specs. I supose that if the PCB has the benifits of solder mask along, with solder paste masks, then a carefull eye may be all that is required.

Perhaps the chips are really "bullet proof", but the specs are on the safe side.

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Toby Seckshund

I agree that the Pb free stuff is awful to use in comparison, fortunatly for repairs we are still allowed to use leaded (its has done me no harm).

You have said something that I had not fully considered, in that the slowness of ramp ( presumably thermal shock problems) is more important than a quick "hit and run"

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