Why not reflow a PCB before placing parts?

Question

A standard reflow method for low-volume assembly is:

Method A

• Apply solder paste (using stencil or syringe)
• Hand-place parts (using vacuum pickup pen or tweezers)
• Oven reflow

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But what happens if one does the following instead? Are there any potential disadvantages?:

Method B

• Apply solder paste
• Oven reflow first time
• Hand-place parts
• Oven reflow second time

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I see no disadvantage to Method B, and one rather convenient advantage:

The solder paste will have already solidified during the first reflow, so after that, part placement cannot disturb/smear the solder paste any more; so, it's easier to get a high-quality finished board (fewer solder bridges or missed solder joints). In contrast, Method A demands greater hand precision, especially for fine-pitch ICs, and really gives you only one shot at the part placement.

Answer 1

Because the method you describe, and it's advantage, is really a disadvantage. Not only is it harder to place smd parts on unevenly tinned pads, it prevents the main advantages of solder paste.

The first is that solder paste holds a piece in place, ever so slightly, before being reflowed. It is a paste of tiny solder beads and flux. The smd parts are suspended in place.

The second being surface tension. When freshly reflowed, free of oxidation and contaminants, there is high surface tension. The parts are not likely to move without external force. Second and third reflows result in lower surface tensions, as well as cold joints.

And the last advantage is capillary action. Between the flux, the surface to surface contact of the pad and the pin, capillary action pulls the pin to the center of the liquid solder. Multiple pins doing this, makes a smd part self center. And as long as you have solder mask between the pins, this is even more effective, of course, only if there is not excessive paste on the pads.

And most importantly, industry leaders have probably tried your method, and having not adopted it, it probably didn't sit well with them. Probably.

Answer 2

Have you ever done any hand-pick-n-place work? You push the component into the solder paste, and it is this paste that holds the components in place when you sneeze over it or otherwise are handling the populated (but as yet not reflowed) PCB.

With the solder already melted you don't only miss the 'gluing'. The flowed solder will have made small 'hills' on all the pads, so now you are into the task of balancing your components on top of two or more hills (small though they might seem for you).

Answer 3

Reflow for many packages just works better when the IC's can sit flatter on the board, and in paste. Try teetering a 100 pin package on the nicely rounded-over edges of hard solder, and see what happens. Your parts will just skid right off the pads before the solder melts.

Less precision than you think is required for positioning IC's, as the surface tension wants to pull the chips into place -- so long as the leads start off somewhere on the pads, which they might not using method B

Lastly, your IC leads will get none of the benefit of the flux in the solder paste, which is activated at a temperature lower than melting (that's why the heating standard is a series of ramp-soaks).

Answer 4

To add to the other good answers, the PCB materials have a limited number of reflows to give before they break down from the heat and begin to cause failures. Your proposal consumes one reflow cycle that could otherwise be used as a repair cycle later. Some materials only provide a couple opportunities to repair something before the heat begins to break things, and those cycles can be very desirable opportunities to have before putting an expensive board in the trash.

Answer 5

If one was to use method B, you could just manually apply solder. Did you ever try that? I can't even nicely solder a SOIC-8 with solder still left on the pads.

• Paste keeps parts in place with its stickiness.
• Flux cleans oxidized metal and helps the soldering process.
• Surface tension self-aligns parts to a certain margin.
• Reflowed solder causes 'bubbles'. In my experience, parts don't want to stay on top of these and glide off.
• More heat cycles on the PCB.
• What are reflow profiles all about...?

If you end up with solder bridges, maybe the paste is wrong/old or you applied too much. Manually reworking it will solve that, although that can be pain on large series or small packages.

If a part ends up misaligned, remove the part with hot air, clean the board carefully and replace it manually. This can happen all the time, for example tombstones on small passives or a QFP/SSOP package that drifts off it's pads (probably not aligned well enough during manually placing parts).