Advice on repairing high temperature PCB - Page 1
Forum section: electronics / repair

author: RFermoselle
author_rank: Newbie
timestamp: October 30, 2019, 03:47:49 pm
content: Hi,Would anyone have experience repairing high temperature PCBs at component level and be able to provide some advice on reflow/soldering best practices?I had four attempts at extracting Luxeon K2 LEDs (now discontinued) from a scrapped RGB PCB to replace on a GLP Impression 90 lighting fixture, but only ended up damaging the component. Please see picture attached to demonstrate how the K2s are mounted. The board is all aluminium. I was using an Antex 760RWK hot air gun at 500C, medium air flow and a good amount of Amtech NC-559-V2-TF flux.I believe I would have to get a heating plate to perform this repair, but if anyone could advise it can be done with a hot air gun alone, I would be much appreciative.

author: andy3055
author_rank: Super Contributor
timestamp: October 30, 2019, 04:07:53 pm
content: According to the site below, there is a replcement:"The LXK2-PW14-U00 has been discontinued. Our SP-05-W5 may be a suitable replacement."https://www.luxeonstar.com/lxk2-pw14-u00-white-luxeon-k2-led-100lm

author: RoGeorge
author_rank: Super Contributor
timestamp: October 30, 2019, 04:17:00 pm
content: A hot air gun is not enough, you need a preheater underneath the board, too.

author: Leighcrx
author_rank: Newbie
timestamp: October 30, 2019, 04:18:08 pm
content: Place board on a hot plate, maybe around 150DegC. Wait for it all to heat up (10 mins) That way all the heat from the hot air gun will not dissapear into heating up the board, less local stress on the board. And turn the hot air down a bit so you do not incinerate the LED's.

author: tszaboo
author_rank: Super Contributor
timestamp: October 30, 2019, 04:21:43 pm
content: Aluminium PCB has crazy high thermal conductivity. It is about 20 times higher than a 4 layer board. Which means, you need to pump in 20 times the power locally, to heat it up to the same temperature. So if you have a hot air gun, that can desolder a part from a PCB at 5% flow rate, you can do the same on an aluminium PCB. But you are trying to desolder LEDs. Those have plastic on top, which melts at lower temperatures than tin.Maybe with special preheating from the bottom, it could be doable, but I doubt it. It is not your technique or your settings, it is just not possible. Not everything which is put together can be taken apart.

author: DaJMasta
author_rank: Super Contributor
timestamp: October 30, 2019, 07:08:03 pm
content: As said, the board pictured is designed to act like a heatsink, so local heating alone will almost never be sufficient. A bottom-side preheater to get the board up to temperature and then a lower actual hot air temperature together would be my approach - the solder is probably melting at 300C or below and the package probably won't stand up to more than that for a long period, so 500C is going to cool your LEDs even if it eventually can get the board to temperature.If you don't have a preheater, you could try other heat sources on the top or the bottom - even something like a hair drier on high could help raise the board temp enough that the local soldering temperature would be high enough to melt, but the controlled way is a preheater with a temperature control. Even multiple hot air guns could help - you're basically just trying to make the board in the space around the part being worked on a high enough temperature that the thermal conductivity of the PCB can't lower the point at which you are soldering low enough to prevent it. To that end, you could try your hot air gun at a lower temperature as your preheater, and you could try a standard iron on two pins at a time (one pass on each side) to heat them up further and more directly to actually desolder it.

author: m3vuv
author_rank: Super Contributor
timestamp: October 30, 2019, 07:16:55 pm
content: ive done similar repairs using an soldering iron heated in an oxy accetaline flame.

author: keymaster
author_rank: Regular Contributor
timestamp: October 30, 2019, 09:22:25 pm
content: Or you could use your hot air to heat the board from underneath until the solder melts. A fresh solder pass with an iron will help . Move the hot air underneath the target led and pick them one by one .

author: Kleinstein
author_rank: Super Contributor
timestamp: October 31, 2019, 08:34:18 am
content: One could try some 150 C from below and some special low meting "solder" alloy like quick chip. These alloys melt well below 150 and would normally mix with the old solder.

author: mzzj
author_rank: Super Contributor
timestamp: October 31, 2019, 09:04:56 am
content: Scrapped pcb and you just need the components? No point of heating from the component side.Heat the alu core pcb on cooktop or even outdoor bbq until solder melts and pick off the components. Bulk extraction: knock the board upside down once solder is melted and you get all the components at once.

author: RFermoselle
author_rank: Newbie
timestamp: October 31, 2019, 11:52:20 am
content: Thank you all for your advice, it's much appreciated.Quote from: andy3055 on October 30, 2019, 04:07:53 pmThanks for the resource Andy. Had a quick look, but it looks like a different footprint and in all honesty this lighting fixture is reaching its 10 years of service and might be retired soon, so extra cost is not justified.Quote from: NANDBlog on October 30, 2019, 04:21:43 pmThat's a great way to put it. I think I'll retreat to the comfort of that sentence before I further damage the confidence in my skills.Quote from: DaJMasta on October 30, 2019, 07:08:03 pmYes, that was a great shout, it reminded me of a heat gun we have; a Titan TTB284HTG with three settings 50/450/600C. Tried it this morning on 450C preheating the bottom of the board from around 10cm distance and tackling the top with the hot air gun, but the most I got was extracting the die with the gullwing leads and dome lens; the thermal pad with the LED tower would stay put and not budge. Please see picture attached of the setup with the heat gun where you'll also be able to see on the board the results of the many failed attempts. The best I got can be seen on two of the pictures attached and it involved some force with flush type cutters whilst heating. It's not pretty and the thermal pad did come with the copper pad attached but the LED lights up when testing with the multimeter.Quote from: m3vuv on October 30, 2019, 07:16:55 pmQuote from: keymaster on October 30, 2019, 09:22:25 pmQuote from: Kleinstein on October 31, 2019, 08:34:18 amThank you for all the tips above. However, I'm afraid the thermal pad on this LED, tightly coupled to the copper pad as it can be, would make reaching it with a soldering iron tip or wetting it with fresh solder very difficult without damaging the component.Quote from: mzzj on October 31, 2019, 09:04:56 amThat sounds great and I might actually try that in the future. A quick look online, however, suggests most cooktops don't go over 500F (260C).And then it got me thinking, I would still need to replace the failed LED on the lighting fixture, which I could extract using some of the bodging I got some practice on, but soldering a replacement on would be another story. It would be easy if I wouldn't need to solder the thermal pad and just apply some thermal compound instead (can anyone confirm?). Anyway, I'll drop this repair for now, as management might question why I'm wasting my time this way on an old piece of kit (not economical, Brexit, blah...). It was good experience though and I'm glad it gave me a chance to have participated in the EEVBlog forum for the first time, you guys were great. Look forward to contributing more.

author: mzzj
author_rank: Super Contributor
timestamp: October 31, 2019, 12:03:53 pm
content: Quote from: RFermoselle on October 31, 2019, 11:52:20 amI'm not sure about UK style cooktops/kitchen ranges but in here at least the older ones with electric/cast iron plates heat to red-hot pretty easily.260C should be enough anyways since it heats the entire board at once.