Double sided reflow soldering - Page 1
Forum section: electronics / beginners

author: homebrew
author_rank: Frequent Contributor
timestamp: April 27, 2018, 07:03:30 am
content: Hi,I need to reflow a double sided board with components on both sides. I've done that in the past but only with some passives on the bottom and all the heavy stuff on the top. Obviously I went with the bottom side first and reflowed the top in a second pass.However, this time, I do have an LQFP64 (an STM32) on the bottom side.Question: Will it fall off during the second reflow? If so, how could I prevent this?Best,homebrew

author: 0xfede
author_rank: Regular Contributor
timestamp: April 27, 2018, 08:37:52 am
content: Hi homebrew,I'm pretty sure that this paper will help you:https://www.jim.or.jp/journal/e/pdf3/47/06/1577.pdfBest,0xfede

author: wraper
author_rank: Unknown
timestamp: April 27, 2018, 08:55:04 am
content: Yes it will fall off. You need to place SMT adhesive under the IC to prevent that.

author: johnmx
author_rank: Frequent Contributor
timestamp: April 27, 2018, 09:00:52 am
content: You can use low melting temperature solder paste for the second pass (side).This way the solder paste of the other side (first pass) will not melt.Of course I don't recommend this for production, only for prototypes.

author: homebrew
author_rank: Frequent Contributor
timestamp: April 27, 2018, 09:07:10 am
content: Thanks for the input.Quote from: 0xfede on April 27, 2018, 08:37:52 amA scientific paper is always a nice answer to a question :-)Quote from: wraper on April 27, 2018, 08:55:04 amClassic practitioners vs. scientific debate - the paper mentioned came to the quite opposite conclusion, even for an QFP208. They deliberately needed to add weight in order to make the components fall.Maybe I'll try it with one of the boards and the post the results here ...

author: Eka
author_rank: Regular Contributor
timestamp: April 29, 2018, 10:05:46 pm
content: Quote from: homebrew on April 27, 2018, 09:07:10 amWhen I reflow a board to remove all the chips, I have to moderately bang the board on something to get them to fall off. Even then some stay put. Most of the small resistors and capacitors will stay put even with hard thumps. You have to scrap or pick them off.

author: Stavos122
author_rank: Contributor
timestamp: May 11, 2018, 08:52:59 pm
content: Solder will melt at your LQFP64 during 2nd reflow but the surface tension will hold it in place. Designers have a formula for this that provides land pad size relative to the part dimensions / weight / number of IOs. Some heavier parts will need adhesive to hold them in place but this is not one of them.You will need to make sure there is sufficient clearance between the bottom side of the PCB and your ovens loading tray. Take a look at a double sided board in this reflow ovenhttps://www.atco-us.com/products/pro-1600-smt-reflow-ovens- third pic from left. Fixture creates space between tray and bottom components. If your board has through holes for mounting screws, you can use standoffs.

author: KL27x
author_rank: Super Contributor
timestamp: May 11, 2018, 09:46:58 pm
content: Dunno what wraper's experience is, but I wouldn't have suspected any problem.I use 28 pin SOIC chip, which is exactly half the body size/weight and less than half the pins to support it. It won't fall off without help.I have some obsolete boards. I occasionally remove and recycle some chips off them if/when needed. I just shot a board at 400C, 100C higher than I normally use, and the chip doesn't budge... even if I hold the board sideways.Removed heat, waited a few secs, and the chip easily falls off with a bump, though.

author: homebrew
author_rank: Frequent Contributor
timestamp: May 19, 2018, 04:58:34 am
content: As promised - the result:In tried it and absolutely nothing felt off during the second reflow. So the STM32 in it's LQFP64 package seems to be absolutely fine in that process.