Desoldering QFN packages soldered by high melting point solder - Page 1
Forum section: electronics / beginners

author: Krotow
author_rank: Regular Contributor
timestamp: January 29, 2023, 05:31:51 pm
content: The general problem - desoldering failed IC QFN packages from (usually) laptop boards @home. PCB have thick ground layer and manufacturers seems used ROHS solder with high melting point. In result chips simply doesn't come off with hot air nor on +280C nor +350C nor +420C. Instead packages begin to melt. So far the only proven way how to cook them off is to drill or sand the chip tops till crystal and then apply a large solder ball with +350C temperature from above. Messy as hell, but works. Soldering fresh chips back with low melting point solder is not a problem anymore (even Sn60Pb40 works with hot air then). Still wish to know - maybe there is less messy way how to deal with these ICs without board preheating?[rant on] I don't have PCB preheater. And will not have it in foreseeable future. At least until affordable preheater portable enough to hide under table will come out. [rant off]

author: tooki
author_rank: Super Contributor
timestamp: January 29, 2023, 09:17:00 pm
content: Quote from: Krotow on January 29, 2023, 05:31:51 pmBoard preheatingisthe answer. A board preheater is, of course, the ideal tool for this (and you can get some for not thaaaat much money*), but you can also just use a hot air gun to preheat the whole board. That way, it's not acting as a giant heatsink when you go to heat the chip to desolder. Move the air around the whole board, with some emphasis on the edges, until it's all hot.*For example,https://www.reichelt.com/lv/de/heizplatte-ir-610-800-w-1-kanal-infrarot-station-ir-610-p105798.htmlfor about €300.Or you could use a €50 toaster oven and use it just to preheat. Heat the board to maybe 160-200C. Just work quickly once you take it out of the oven!

author: Krotow
author_rank: Regular Contributor
timestamp: January 30, 2023, 12:13:26 am
content: Sigh. Seems I must find some portable and "invisible" preheating solution then. Money for it is not a big problem, but nosy relatives, who already rant me from my socks off about already existing repair hardware in my "repair" room, are. Can't go away from here now due to rent cost in/around my city. Otherwise preheater will be least of my problems.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 30, 2023, 02:02:10 am
content: Even an air dryer could help heat the pcb ?? done that for a qfn-52 with an huge pad under itIt had sh@$@$# rohs solder, and with the right flux paste type, it helped a lot with an hot air desolder gun on top of it

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 30, 2023, 04:48:12 am
content: Blast the entire board with those 2000w heat guns (Not too close, or it'll burn plastics etc), after 1-2 minutes it should be pretty hot, now shoot the qfn with the soldering gun and should come off easily.

author: Krotow
author_rank: Regular Contributor
timestamp: January 30, 2023, 07:53:49 am
content: Particular board have CPU and SODIMM socket at opposite side of board nearby location of beer soaked (and corroded then) battery charger chip I wanted to replace. Random heat gun without temperature control from bottom then become not so desirable approach. Although I got the idea about minimizing temperature difference between board temperature and solder melting temperatures. Seems heating board to +110..120C will help in great degree without melting all things at opposite side of board. In end ordered YIHUA 853A preheating station - seems large enough to cook laptop and smaller boards and small enough to hide when not needed. I don't expect to replace large BGA chips anyway.

author: tooki
author_rank: Super Contributor
timestamp: January 30, 2023, 09:51:26 am
content: Quote from: Krotow on January 30, 2023, 07:53:49 amEh, it's not as bad as you think. Hot air isn't a precision process, since even small differences in distance, air speed, etc. change the actual temperature at the board significantly. For preheating a board at a distance, a big heat gun is fine. The point, as you said, is to get the whole board to a temperature much closer to the melting point, without actually exceeding it. I have often used my small 700W hot air unit to do this, but since it's small I have to be closer and move it around a lot more. A bigger one would simply let me do it from farther, with less frantic movement.But your preheater purchase is the right choice since you plan to do this kind of stuff frequently.Honestly, I can't understand why big-name preheaters are so expensive; they're not precision devices, either, and it's not as though resistive heating is an expensive technology... Let us know how the Yihua is!

author: wasedadoc
author_rank: Super Contributor
timestamp: January 30, 2023, 12:58:06 pm
content: You could also tryhttps://www.chipquik.com/store/product_info.php?products_id=210001

author: janoc
author_rank: Super Contributor
timestamp: January 30, 2023, 02:08:43 pm
content: Quote from: wasedadoc on January 30, 2023, 12:58:06 pmI wonder how would you get Chipquick under a lead-less QFN package, especially if it has the central thermal pad.