How well do parts self center during reflow? - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: April 30, 2014, 08:16:01 pm
content: Hi,I'm going to try my hand at oven reflowing some boards soon - how careful do I need to be when placing parts they are centered or will the reflow process do a good job of self centering them?Also, can you count on self centering with components that require good accuracy in their board placement like a switch that will come through an enclosure. Will a tactile button self-center well enough for this type of accuracy?Thanks,Alan

author: Sebastian
author_rank: Regular Contributor
timestamp: April 30, 2014, 08:39:08 pm
content: If the footprint is right and the PCB is good quality with good soldermask it works fine Otherwise maybe, but probably not as reliable.

author: sacherjj
author_rank: Frequent Contributor
timestamp: April 30, 2014, 08:52:52 pm
content: The heavier the component relative to the solder area, the less it will self center. 0402 passives with the right pads really line up. For your switch, it really depends on how small it is.

author: mazurov
author_rank: Frequent Contributor
timestamp: April 30, 2014, 10:23:37 pm
content: It also depends on the solder alloy. Liquid 63/37 has the highest surface tension. SAC305 won't center parts larger than 32 pin TQFP (but will center QFN of any size due to large center pad). SnBi has the worst surface tension.On the other hand, wrong pads will cause your parts move in all 3 dimensions, even ones being perfectly centered before reflow. Oversized 0402 pads will cause tombstoning (surface tension will be your enemy here). Uneven heating is another cause for mis-alignment - if you have a choice use a skillet, not an oven.

author: alank2
author_rank: Super Contributor
timestamp: April 30, 2014, 11:10:48 pm
content: I keep seeing the idea of skilled and not oven - what are the advantages?

author: dannyf
author_rank: Super Contributor
timestamp: April 30, 2014, 11:34:19 pm
content: QuoteFloating on molten tin with surface tension will do that for you automagically.

author: gxti
author_rank: Frequent Contributor
timestamp: April 30, 2014, 11:49:54 pm
content: SAC305 has higher surface tension than SnPb, not lower. 0.567N/m vs 0.400 for Sn63. It's readily noticeable in hand-soldering, as SnPb has less tension but wets better while SAC wets less readily but bridges less often.Numbers from here:http://www.ipcoutlook.org/pdf/weight_limit_qfn_smta.pdf

author: mazurov
author_rank: Frequent Contributor
timestamp: May 01, 2014, 03:03:46 am
content: I was noticing better centering with 63/37 than 305. Maybe density plays a role?

author: alank2
author_rank: Super Contributor
timestamp: May 01, 2014, 12:37:25 pm
content: Do you ever apply any flux on its own to certain tighter pad areas before stenciling? Is that ever necessary?

author: JuKu
author_rank: Frequent Contributor
timestamp: May 16, 2014, 09:02:29 am
content: Quote from: alank2 on May 01, 2014, 12:37:25 pmNot necessary, the paste has its own flux. If you need pre-flux, you have a serious issue with your boards.

author: kayvee
author_rank: Regular Contributor
timestamp: May 16, 2014, 09:41:24 am
content: Depends on the size of the switch, really.If you are referring to the fairly common 6X6mm SMD tact switches then no, they definitely will not self center in my experience.

author: uoficowboy
author_rank: Regular Contributor
timestamp: May 17, 2014, 03:43:08 pm
content: Quote from: mazurov on May 01, 2014, 03:03:46 amMy experience has been that 63/37 solders do better than SAC305 as well. Maybe the flux is playing a role somehow? Not sure...

author: zapta
author_rank: Super Contributor
timestamp: May 17, 2014, 05:23:13 pm
content: Quote from: alank2 on April 30, 2014, 11:10:48 pmThe argument given in iantube video is that it's easier to fix/move the components if something goes wrong.