How do you reflow both sides of a board? - Page 1
Forum section: electronics / projects

author: Jelby-John
author_rank: Contributor
timestamp: May 01, 2015, 10:19:31 pm
content: I was just watching Dave's teardown of the Yamaha mixer and it dawned on me that I have no idea how you reflow solder to the second side of a board without the first side's components falling off. I realise that this might be a stupid question and I'm probably missing something obvious but... how?I've done single sided reflowing before but always avoided mounting any SMD to the underside as I figured it must be a high-tech assembly job. Can someone shine a quick light on that for me?Thanks ever so.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 01, 2015, 10:27:33 pm
content: I do double sided PCB's daily. I thought at first it would be a pain, but its really just putting the PCB in the oven twice. It helps that I am also the one designing the PCB so I am sure have one side with only small and light components. The 2nd side is used for inductors, electrolytic caps, etc. If the profile of the oven is carefully controlled the 1st side will never flow but even if it does, the surface tension of small SMD's in a puddle of solder is enough to keep it in place.So far I have never had anything fall off or mis-align after a few hundred PCB's. Someone in real production may have more detailed notes, my experiences are good, but limited.

author: Jelby-John
author_rank: Contributor
timestamp: May 01, 2015, 10:30:35 pm
content: Thanks rx8pilot, that makes sense. Simpler than I thought.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2015, 10:53:25 pm
content: Yeah, either that or glue.Glue is especially required for wave solder (which would just wash everything away), or bigger parts.Tim

author: marshallh
author_rank: Unknown
timestamp: May 01, 2015, 11:06:01 pm
content: Even surprisingly large components like USB ports, TSOP flash and sd slots can stick on the bottom by surface tension.Lead-free solder has slightly higher strength.Of course, be careful not to bump while reflowing...

author: redshift
author_rank: Contributor
timestamp: May 02, 2015, 04:23:36 am
content: You could also reflow it twice with two different solders each with different melting points.I have no idea if this process is ever used in practice though...

author: rx8pilot
author_rank: Super Contributor
timestamp: May 02, 2015, 04:50:27 am
content: If the placement and temp profile is reasonable, only the fresh solder will flow even if it is the same on both sides. The top of the PCB will be hotter while the bottom is blocked from the heat source.My oven, while not amazing, is a tightly controlled convection oven. Hot plates and toaster ovens may be substantially more challenging.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 02, 2015, 05:41:10 pm
content: Definitely need the double sided PCB. Not just for density, but for proximity. In the delicate analog areas, with a central , .5mm Pitch QFN its hard to get all the parts that need short traces close enough. Putting components on both sides allows the critical paths to be shorter.In power supplies, I put high current switching on one side and all the delicate control signals on the other with a solid ground plane in the middle. This offers a significant performance improvement in addition to density. Since going to double sided designs, I have added many more components and still have room left for future updates without having to make the PCB bigger - which would require the mechanics of the device to change. In addition, I have been able to improve the performance of my circuits.

author: ajb
author_rank: Super Contributor
timestamp: May 02, 2015, 06:34:32 pm
content: Quote from: blueskull on May 02, 2015, 01:27:23 pmIf you're using a two layer board, then any component placed on the bottom is a compromise between achieving better proximity and reduced parasitics and carving up your ground plane. I'd agree that for most two layer designs there'd be very few components that you'd want to put on the back side unless it's something that's very size-constrained and you don't care too much about signal integrity. Doing things like putting decoupling caps underneath their ICs instead of around, or moving one or two parts of a switching converter to the back can be a big help in tightening up a layout, though.If you've got four or more layers it's an entirely different story.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 02, 2015, 06:49:41 pm
content: 4 layers minimum for my 2-sided layouts. Agreed, with only 2 layers, there is not nearly as many options.

author: Corporate666
author_rank: Unknown
timestamp: May 02, 2015, 07:46:47 pm
content: Most of my PCB's are 2-layer and most of them have parts on both sides. I have PCB's densely packed on both sides where the components would not physically fit on a single side.Ground planes are a rare luxury for my stuff.People talk about signal integrity but I have not found it to be a problem at all, even with signals up to a few Mhz. If you are working on really high speed stuff, then it's obviously much more important. I also have no problem holding SOIC-8's, SMB's, DPAK's, MELF's and 10mm^2 inductors in place on the 1st side of a double sided board as the 2nd side is reflowed.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 02, 2015, 07:51:00 pm
content: Quote from: Corporate666 on May 02, 2015, 07:46:47 pmGlad you said that, I was always to chicken to try and lay out a PCB with heavy parts on side 1.