Reflow oven question: Anyone ever made double sided pcb's?

Don M

Has anyone here ever built and "baked" a pc board with components on both sides? I've done quite a few of single sided boards but never tried a double sided board. Was wondering how well the parts stayed in place and if you used anything besides the solder paste to hold the parts in place.

What orientation did you set the boards in the oven? Parts on top / bottom or edgewise left / right?

Thanks.
Don

Phil Pilgrim (PhiPi)

I've done that -- nothing other than solder paste and surface tension to hold things in place. The side with the fewest/lightest parts should be done first.

-Phil

Don M

Phil Pilgrim (PhiPi) wrote: »

I've done that -- nothing other than solder paste and surface tension to hold things in place. The side with the fewest/lightest parts should be done first.

-Phil

So you baked one side first then placed parts on second side then baked again? I guess I was envisioning baking both sides at same time.

T Chap

A glue dot is used on parts to hold the on the bottom. Typically in fabrication there will be a glue dispenser that puts a small dot of glue under the parts that will be on the bottom on the next oven pass. There are different methods to apply glue. Some use a stainless stencil just like applying the solder paste, just run the glue first. There are also machines that apply a dot of glue.

SRLM

I've made double sided PCBs without any problem. I use small right angle brackets from Home Depot as board holders. I always do it in two passes:
1. paste and place the bottom, cook it, let cool
2. paste and place the top, cook it, let cool

Glue isn't necessary if you're willing to go in two passes, but is if you go in one pass. The un-heated surface tension of solder paste is not enough to stop bottom components from falling.

You can try it for yourself quite easily: just take a one sided PCB and heat it up upside down. You'll find that everything stays in place.

Phil Pilgrim (PhiPi)

Yup: two passes, no glue. 'Didn't have a problem with parts dropping off the bottom.

-Phil

jmg

Don M wrote: »

Was wondering how well the parts stayed in place and if you used anything besides the solder paste to hold the parts in place.

Unflowed paste is going to be quite poor at holding parts - most uses just reflow the parts (2 passes), so you then rely on the flowed solder surface tension - much better at holding parts in place.
There is still some upper limit of weight / wetted area on the bottom side parts, and that is worse if the boards move on a bumpy conveyer.

If you really want a single reflow pass, you could try warming the paste to melt the flux, to better 'stick' the parts.
Then you have a race between gravity, paste burnoff, and wetting, in the bottom reflow.
I'd guess more tomb-stoning could be one outcome and largest-size would be smaller .

Cluso99

Professionals use the red glue dot method.

For hobby or low volume 2 pass, i do the first side (least and smaller components). Then screen and then place on an old bare pcb in the oven, or wrap the underside in foil, orboth, then reflow. this prevents the bottom side getting so hot to melt the solder. Works for me.

Duane Degn

Before hearing about the solder being able to hold chips to the bottom of the board, I thought using lead free solder on one side of the board and leaded solder on the other. I figured I'd set the parts using the lead free solder first and once the board cooled, add leaded paste to the second side. The lead free solder shouldn't melt during the second pass if the temperature of the oven only gets hot enough for the leaded solder to melt.

I have yet to oven solder myself but I expect to do so in the next few days. (I don't expect to try the two different solder types technique anytime soon.)

T Chap

If it is just for a few one-offs and the parts are too heavy to stay attached, you can bake one side then put a dot of epoxy on one or more sides of the component to hold it to the PCB. Then flip and run the other side once the epoxy has set.