[SOLVED] Would it be better to place components on top or both sides of the PCB board?

Hi,

Would it be better to place components on top or both sides of the PCB board?

It is certainly OK to have components placed on the bottom of board, and there are many resistors, capacitors and inductors placed like this commonly.

The question is with ICs, especially BGA and QFN packaging. If a BGA or QFN chip is placed on the bottom side, then since ordinarily most other BGA/QFN chips are placed on the top side, then both sides would now have BGA/QFN chips. Can they still be soldered by machine in this situation? If it is possible, would cost be increased if BGA/QFN chips are to be soldered on both sides, and what is the technology to do both-side BGA/QFN soldering?

Additionally, what is the current technology for soldering BGA/QFN chips when they all lie just on one side? Is it by reflow soldering or other ways?


Bob

re: BGA on both sides

We often have BGA / QFN as well as other parts placed on both sides of a board without issues. I was worried at first but the assembly house said no problem. It is my understanding that each side is processed individually and when the second side is processed the reflow temperature is controlled well enough to not melt the solder holding the parts on the bottom while reflowing the top.

As for the cost I don't believe the cost would be different just because there are BGA parts on both sides. I think it just based on the fact both sides need to be reflowed and the pick and place time for each side. We do low volume designs so we are not as concerned with price so there may be other issues I'm unaware of.

Ray

re: BGA on both sides

Ray,

Thanks. I will consult assembly house for this.

Bob

re: BGA on both sides

Should be fine from the Assembly point of view. But make sure there are no thermal related issues (airflow) while assembling BGA's on either sides of the Card

re: BGA on both sides

CKS,

This is a very important point. Thanks for reminding.

Bob

re: BGA on both sides

Also remember that the thermal dissipation numbers for a part mounted on a pcb assume there are no heat generating parts are mounted below. This has gotten me in trouble a couple of times.

Ray

re: BGA on both sides

Ray,

Many chips uses a thermal pad and thermal vias to dissipate heat to internal GND planes(s). In such case isn't the GND plane(s) a very big heatsink? Could you give some examples on the problems you encountered?


Bob

re: BGA on both sides

Bob,

Well we had some SDRAMs on one side and a power supply on the other. When I did the thermal analysis on the power supply the additional heat from the SDRAMs was not taken into account. I assumed the power supply was being heatsinked by the copper planes, which it was, but the SDRAMs were adding heat into the board at the same area causing the both the SDRAMs and power supply to run much hotter than planned for. Of course this seems obvious now but at the time I was just looking at the supply as if it was mounted on a single sided board and calculated the temperature rise based it's components forgetting about the heat from the SDRAMs. In the end I was ok but not by much.

We have also packed so much circuitry into such a small area using both sides of the board that we had to treat the area it as a lump heat source with parts on both sides contributing to the total power dissipation.

Just be aware that it's easy, at least for me, to overlook the heat generated on the bottom of the board and how it interacts with the top side circuitry.

Ray

re: BGA on both sides

Ray,

This is a very pertinent example considering the amount of heat SDRAM generates. Thanks for sharing.

Bob