Reflow double sided PCB

[BuffaloEngineer]

If you have a double sided PCB and want to reflow surface mount components on both sides, I've been told that you should really only put large components on one side - save the bottom side for small components like resistors and capacitors.

Is that a best practice or a requirement? Does anyone have any experience with this reflowing double sided PCBs with large parts on both sides?

 

[jbeng]

Most double-sided boards I've seen are laid out with small parts like that on the bottom. Those small parts are glued to the bottom side of the board, the top side gets populated with large components and reflowed soldered, prior to wave soldering the bottom side. I don't think it's required, it just seems to work better that way...

Jeff

 

[Mr RB]

One reason they do that is because they glue fix the large ICs and solder them, then when they turn it over to to the majority of smaller parts the glue holds the ICs and they don't fall off even though upside down.

If it's for hobby SMD soldering you dont need the glue, just use a frypan at about 150'C and use hot air on top from a heat gun etc, you can solder the top parts without the bottom getting hot enough to melt.

 

[Eclipze]

I've got a small reflow oven off ebay (from China). I reflow double sided boards, but those with small components on the bottom... 0805, SOT23 etc... I reflow the bottom side first, then populate the top side and reflow a second time. The bottom components don't move or fall off.

 

[BuffaloEngineer]

After digging around on the web, I found the attached PDF. They have an equation on page 2 for what components should work:

C(g) / P(a) ≤ 30

Where C = components weight in grams; P = total area in square inches (g/in² must be ≤ 30 for second-side mount).

I've got a small reflow oven off ebay (from China). I reflow double sided boards, but those with small components on the bottom... 0805, SOT23 etc... I reflow the bottom side first, then populate the top side and reflow a second time. The bottom components don't move or fall off.

I was looking at reflow ovens on ebay - does it save you a lot of time? I always thought the real time savings came from having a pick and place machine.

 

[Eclipze]

It definitely does save time. One double sided board used to take me 29mins. Using solder paste, it brought the time down to 19mins each board. It would be quicker if I had a stencil to do the paste, rather than dot by hand. But for small productions runs it's quicker than using an iron (and I've had plenty of practice). Results are quite good, despite the reflow oven having a really pathetic bang-bang controller for the IR and fan, and the LCD menu system is very unrefined. I still can't get over how the timer has such a large error, where it might say 4 minutes expired when it's actually 6 hahaa...

 

[andy257]

I work in the manufacturing industry and ive not heard of using "Glue" to keep components held in place. The majority of our products are larger than average size and we tend to use two types of solder paste. One low temp and one high temp. This way none of your components move while reflowing the opposite side.

you can get conductive appoxy (is this the "glue" you speak off?) which is similar to solder paste only once reflowed its permanant and does not reflow once its cured.

Andy

 

[dknguyen]

I heard the small component side of the board is supposed to be reflowed first. Then you take it out and populate the other side of the board so when you put it back in, the small components are held on by surface tension of the solder alone.

 

[Eclipze]

The automotive supplier I worked for uses glue all the time. They robotically dot pink glue under all the components prior to placement. I always thought it was for just wave soldering surface mount components, but I guess it comes down to production yield/quality/rework. It does make it a pain to scavenge parts of scrap boards though hahahaaa..

 

[Mr RB]

Yeah pink or red glue dots. It is more of a 1990's technology though, you don't see it as much now.

 

[jbeng]

The contract mfg I formerly worked for often glued the bottom side parts on, but they used screened-on glue; they didn't have a robot to apply it.

Jeff