Is it hard to do 2 sided SMD PCB in oven?

Question

As far as I am aware, and please correct me. If we have a 2-sided PCB with SMD components on the top and also bottom, first we shall put components on the top side and solder them and then on the bottom side and solder them. It could be done the other way around but the thing is that, it will be done on one side at a time. The soldering could be done by hand if the component side allows or we could use solder paste with hot air gun.

If we are to do a complex board then we would be using a reflow oven. For prototype board the components would be placed by hand and then the board would be put into the oven. For manufacturing at industrial scale the component placement would be done by a machine.

Now the question in my mind is, is it true that if we have to do 2-sided soldering of such a PCB it would be that once the one side has been soldered in the oven and we place components on the opposite side and put into oven again, the components would fall off from the first side due to heat in the oven? If this is true, how is this mitigated?

Answer 1

As other answers mentioned the light parts are usually not a problem. For heavy parts there is special heat curable adhesive that will ensure that those stay in place. Adhesive can be placed either under the component if there is no bottom pad or can be placed at the corners (for BGA components).

Good SMA house will have machine that dispenses adhesive. For manual process it is sold in syringes same as solder paste and can be (very carefully) applied to the board.

This type adhesive is cured by heat and allows for double sided assembly where you have heavy components on either side. The process will still remain the same though - first reflow one side then reflow the other side. Of course this also requires attention to components maximum temperature. If you have components that require low temp those should be all on same side and that side should be last to reflow to ensure that you don’t damage them by overheating.

Credit: images lifted off google search results. All credit goes to their rights holder.

Answer 2

No, it’s generally not a problem. You can solder the light parts first and then flip it and solder the rest. The surface tension of the solder will hold small parts onto the board.

In the unusual case where you might have a heavy part or two on the bottom on the second cycle you can add adhesive or solder it later by hand.

'Heavy' relates to the relationship between pad area and weight, with a rule of thumb limit of about 30g/in^2 of pad area. So, for example, a 1206 resistor with 0.005in^2 of pad area has a limit of about 0.15g. The actual mass is 10g/1000 or 0.01g each.

See this accepted answer for references.

Answer 3

Many assembly houses can do this and have different methods. A cheap and dirty way to do this at home would be to use different solder materials with different melt points.

Answer 4

The correct answer is to check with whomever is doing your SMT assembly and ask them.

Double-sided boards are very common. But your own vendor has to be setup to do it.

Answer 5

It is done as you mentioned using adhesive tape on already soldered side.

Also, components should be selected carefully and heavy component should be put on 2nd side if possible.

[EDIT] : My mistake only some low cost operations do the tap on bottom side to support heavy parts. Typically reflow profiles is carefully curated and SMT JIG is used after one side is done to hold up components. This supports heavy parts.

_{Image source: My own photo}

Answer 6

We used to do reflow on top and then wave on the bottom.

On the bottom side, the pick and place machine would stick the components down with glue. The solder wave would do both the SMT and the PTH.

I don't remember the specifics but when we moved to double-sided reflow (to support BGA devices) there was some sort of upgrade that went on in the factory.