catch-22 rework? - Page 1
Forum section: electronics / projects

author: vivi-d
author_rank: Regular Contributor
timestamp: October 06, 2020, 01:45:59 pm
content: Hello,I need some advice on how to rework sensitive SMT parts that are very close to other sensitive SMT parts.As you can see in the picture, this inductor is surrounded by electrolytics. I tried to solder this inductor to the board with solder paste and a heat gun, but one of the caps blew up.My question is how can I rework a component without overheating the components right next to it?? What is the technique/strategy?Thanks!

author: Kalvin
author_rank: Super Contributor
timestamp: October 06, 2020, 01:48:31 pm
content: Use Kapton tape to protect the area that you do not want to have the heat.

author: Shock
author_rank: Super Contributor
timestamp: October 06, 2020, 02:09:02 pm
content: You can use kapton tape or a heat shield to deflect airflow into the target area. Preheating from the underside helps as well as the reflow temp is reached quicker without having to saturate the board from top down.Depending on tip size and pad access desoldering tweezers or two irons. If you can apply solder to pads a low melt alloy can assist and lastly flux generally allows an easier reflow.Another point I'll bring up, removing or replacing those caps is not a big deal either so sometimes it's easier to remove a few parts if access is difficult, especially if they are low cost. Just practice on scrap boards first.

author: vivi-d
author_rank: Regular Contributor
timestamp: October 06, 2020, 02:45:43 pm
content: I'll give it a shot, thanks for the tips

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 06, 2020, 04:37:23 pm
content: 2 or three layers of aluminum foil, loosely coupled with air pockets in between. and fixed with a few pieces of kapton tape work well I've heared.Pre-heating the whole board from the underside with a moderate temperature is also a common thechnique.With rework on fine pitch boards, the final soldering after cleanup is often done with solder with a lower melting temperature.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 07, 2020, 12:29:01 am
content: Short of protecting the surrounding components with some means, as said above, two other thoughts:* Is the inductor not solderable with an iron instead of hot air? I've soldered this kind of SMD inductors with a soldering iron many times. Unsoldering may be a bit trickier, but doable with some patience if you're ready to sacrifice the original inductor anyway.* If you absolutely need to use hot air, another approach would be to unsolder all surrounding caps first, and solder them back afterwards. May look a bit much, but it's really faster to do than to waste a lot of time trying to figure out a proper approach for leaving them in place.

author: DaJMasta
author_rank: Super Contributor
timestamp: October 07, 2020, 05:32:03 am
content: I'd probably do the kapton tape as a shield on other parts, then heat with the heat gun and touch an iron to the pads. An inductor that big is going to be on big pads with big traces and has a huge amount of thermal mass, so the direct heat injection of the regular iron on a pad can make a pretty effective spot-heating effect on the part you want to move. Similar to using a preheater, but using the normal hot air as your preheater. You could also potentially do the same with any pads/vias through to the other side if there are some quantity - this works especially well on ICs with a power pad under them, since it's otherwise the most insulated place from a stream of hot air.

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2020, 09:00:42 pm
content: Use a soldering iron instead of hot air. Nothing there requires hot air.

author: tooki
author_rank: Super Contributor
timestamp: October 08, 2020, 02:24:10 pm
content: Quote from: vivi-d on October 06, 2020, 01:45:59 pmFYI, going by the picture, you way, way overheated those parts. Like, not just a little bit. You can see that from the black bubbling on the inductor, the melted plastic bases of the caps, etc.Inductors are a pain, due to their huge thermal mass. You need some combination of a heat shield, heat from below, or do a much longer “presoak” at a moderate temperature, so that you only need a comparatively short blast of higher heat to get to a melt. My guess is you have a huge ground plane there too, so that not just the inductor’s mass is coming into play. This really speaks for using a generous presoak and heating from below. Then go in with an iron from above to actually solder the thing down.P.S. At this point, you must consider all of those caps as having been wildly overheated, so you must replace them. I’d remove them, deal with the inductor, and then install the replacements.

author: richard.cs
author_rank: Super Contributor
timestamp: October 08, 2020, 03:30:15 pm
content: Quote from: tooki on October 08, 2020, 02:24:10 pmI'd probably change U8 and D5 at this point too unless this is just a prototype where you don't mind poor future reliability.

author: chrisl
author_rank: Regular Contributor
timestamp: October 08, 2020, 08:45:38 pm
content: A good soldering iron will do the job, no need to use a heat gun for this one.If you must use hot air, use Kapton tapes as heat shields.