Preheaters value for working with multilayer board repair/rework. - Page 1
Forum section: electronics / general

author: cdev
author_rank: Super Contributor
timestamp: August 10, 2017, 11:06:17 pm
content: The other day I was trying to make a small change to a mutilayer PCB - removing an antenna- its connection was anchored by a peg through a multilayer PCB. It needed a lot of heat to get it out.Anyway, the device now seems nonfunctional. It was a leadfree solder device and so the solder required much more heat to melt, and I think that heat screwed up the vias or some internal connections in the board.Would preheating at a lower temp than the melting point of the lead free solder and then just heating the peg briefly to get the antenna off, been the way to go?

author: thm_w
author_rank: Super Contributor
timestamp: August 14, 2017, 10:00:36 pm
content: Quote from: cdev on August 10, 2017, 11:06:17 pmYes, but its not clear what you did. Did you heat the whole board up to reflow temperature? Maybe a photo would help.Usually you'd preheat to 120C or so.

author: cdev
author_rank: Super Contributor
timestamp: August 14, 2017, 10:22:45 pm
content: This is a GPS module which was basically never working well, it was unable to get a decent fix and it had a high level of noise. I think its a bad design because when I removed the antenna there were traces right underneath it. The "ground plane" was broken up by traces. It was free so I decided to see if I could fix it by removing the passive antenna and connecting a SMA or pigtail to it so it could use an external antenna. But I had to heat it for a while, until the solder melted and I was able to pry the antenna off. That took around a minute. Much too long.So after removing the antenna the "fix" light is on all the time and it doesn't receive anything but noise.Basically itdoesn't see any signalsat all, even when an antenna is connected to the pins, BUTstill has the high level of noise.At that point I decided its an issue for another day and put it in my broken/junk box. I think the module is quite possibly still okay but would need to be removed and put on another board.What I was thinking is that theprolongedheating of the pin may have fried the vias around the passive antenna connection,shorting it out, but leaving the noise thats being induced or capacitively coupled into the front end by the traces.

author: thm_w
author_rank: Super Contributor
timestamp: August 14, 2017, 11:04:52 pm
content: Soldering temperatures will certainly kill that lithium battery. I've never had a via short as you are describing though.The external antenna doesn't need any coupling/series components soldered in right?

author: cdev
author_rank: Super Contributor
timestamp: August 15, 2017, 12:47:10 am
content: No, it was a passive antenna that was/is supposed to be soldered above a ground plane that fills up the entire area beneath it, except for the hole, but there were several traces there and the ground plane if it existed must have been connected by vias to other layers which is asking for trouble.I didn't test the battery again but when I tested it earlier it appeared to be almost dead. I should just remove it.It was very hard to get the ceramic antenna off without breaking something physically.(I saved it) I can reuse it.I should have used a different tip on my soldering iron. Something with a bit more mass. I didn't, I used a basic screwdriver shaped tip. I had to bring it up above 300 to get the solder to melt.300 is awful hot. That was probably because of the low thermal mass and my having to apply heat longer.I think perhaps a preheater would have made it easier to get in and out with less time wasted essentially reheating the antenna peg- ?

author: eKretz
author_rank: Frequent Contributor
timestamp: August 15, 2017, 02:27:42 am
content: Yes, preheating is very useful for things like that. I've had no end of trouble soldering some boards in the past that turned out to be easy peasy after preheating. It's also nice because preheating is usually done on the entire PCB, so there's less stress put into the board from localized heating.

author: cdev
author_rank: Super Contributor
timestamp: August 15, 2017, 02:31:53 am
content: Also it would be useful to use for fast reflow of very small boards when needed. Controllable and repeatable.

author: cdev
author_rank: Super Contributor
timestamp: August 18, 2017, 09:45:02 pm
content: Chip Quik sounds super useful. I definitely need to check that out. I also should have used a larger tip with more surface area next to the joint. I think the module is still okay, its just the board that is not working but then again it never really did.I managed to get a good deal on a "digital hot plate" but I have yet to check it out. Its exactly that, a controlled digital hot plate.Likely will also be useful for reflow.

author: cdev
author_rank: Super Contributor
timestamp: August 19, 2017, 01:08:08 am
content: Sounds a bit like the slight expansion liquid water goes through as it turns into ice, the bane of homeowners and roadbuilders everywhere.Its weird but I could actually see something changing about the material. The vias made a little circle around the peg and the optical quality of that area changed slightly. I had a really bright light shining on it.265.. Thats good to know. (taking notes here, thank you for that info)My previous method of reflow uses these two little bimetal contact thermometers I have but they are not so accurate. (or need to be calibrated)Now that I have a thermocouple-capable multimeter, I need to buy one. (a k-type thermocouple)