removal of 7-segment displays from PTH board - Page 1
Forum section: electronics / repair

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: August 09, 2024, 10:20:34 am
content: I have a pcb with 8 x 7-segment displays, two colour, single digit, located tight together in a row, with 5 pins on the top edge and 5 on the bottom edge. They have all got failed segments probably due to being overdriven to achieve the brightness to be visible through the tinted glass panel they sit against. I am a bit wary of trying to remove them and wonder if anyone has any tips and suggestions. I have a Pace MBT with desoldering wand, and a hot air gun (that wanders a lot in temperature) and am fairly experienced with desoldering small components. The pads on the solder side are really small so I worry about damaging them and/or pulling out the through-hole plating due to so many pins.Regards, BT

author: jpanhalt
author_rank: Super Contributor
timestamp: August 09, 2024, 10:52:28 am
content: There are several possibilities. If the leads are accessible, cut them, then remove the pieces from the board. They can be cut either with snips or a small burr in a Dremel like tool. Hot air is another possibility. Low-melting solder is another. I usually go the cut and remove approach, if not surface mount or I don't need to preserve the piece being removed.

author: tooki
author_rank: Super Contributor
timestamp: August 09, 2024, 11:21:37 am
content: Quote from: BurningTantalum on August 09, 2024, 10:20:34 amAm I understanding correctly that your only concern is damaging pads/PTHs?I assume by “desoldering wand” you mean a vacuum desoldering handpiece? I assume it is continuous-vacuum (i.e. vacuum continues as long as you hold the trigger, as opposed to a solder sucker where you get one finite “shot” of vacuum)?If so, then you just need to learn proper desoldering technique. With proper technique, you won’t damage the PCB. When done properly, components should practically fall out of the holes.I’ve found no better tutorial on this than this video manual from Pace. It’s technically for one of their old handpieces, but just skip the parts that are tool-specific and look at the technique shown, because it works with any continuous-vacuum desoldering system.

author: Haenk
author_rank: Super Contributor
timestamp: August 09, 2024, 12:36:34 pm
content: A picture would help.If it's a dual layer board, touch up all the solder joints with fresh solder, better add some extra, that will ease extraction. If it's a private repair, use 60-40 solder. The desoldering should be fairly easy with fresh solder.Use a good amount of heat for the desoldering iron, I run my pace at 460 oC most of the time. Need to be quick though. Don't use any pressure on the plating, only initially on the cold solder. The plating should not suffer (unless it's one of those awful Phenol-based single-sided hard paper boards)

author: tooki
author_rank: Super Contributor
timestamp: August 09, 2024, 01:23:54 pm
content: 460°C? Yikes. What condition your desoldering tips are in, when run like that?I have a Pace desoldering iron, too, and usually run it between 350-400°C. I just preheat the board with hot air if needed, and/or add auxiliary heat with a soldering iron.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: August 11, 2024, 03:38:39 am
content: Apologies, the desoldering tool is a Pace MBT (I was posting a related MetcalTalon query at the same time)The displays are in 2 groups of 4. It doesn't seem possible to cut the pins above the pcb due to the proximity of other components (see pic) It may be possible to Dremel across the width of the group, with a shim of steel slid in underneath the packages to protect the traces underneath, which would leave me with groups of 5 pins which I would be more confident in attempting to remove.I have a replacement pcb so have nothing to lose but would like to repair this one as a spare.Thanks for the link, Tooki. I have found that if the clearance of the lead in the hole is sufficient then the Pace tool or a hand desolder tool are relatively easy to use with a 'wiggle' to leave the lead centred in the hole and not touching the sides, but sometimes the lead is almost an interference fit in the hole and they are much more likely to give trouble.Incidentally, whilst trawling around I found details of a 'solder fountain', a tool/system I have never heard of or seen before.Thanks for the suggestions.regards, BT

author: jpanhalt
author_rank: Super Contributor
timestamp: August 11, 2024, 08:57:31 am
content: Considering that picture, I would use a solder sucker and low temp solder followed by hot air. Hot air alone might work, but I would be worried about those SMD resistors falling off.

author: Haenk
author_rank: Super Contributor
timestamp: August 11, 2024, 07:02:07 pm
content: I'd just add some fresh solder (60:40 with flux) to add somewhat more "substance" for the sucker and use the MBT. If there is room to wiggle that should work out fine, I don't expect some nasty ground planes there. Don't cut or dremel things for now.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 11, 2024, 08:58:47 pm
content: Hi,I would carefully remove as much as possible of the original solder using de-soldering braid (example solderwick).I would then apply, very low melting point solder:https://www.chipquik.com/datasheets/REMKIT.pdfThis solder is meant for SMD rework, but it also works with thru-hole parts.You can also find 138C solder (58% Bi 42% Sn). This will also help and is much cheaper.I would then gentle heat the board with a hot air and the 7-segment displays will fall out of the board.This will minimize the risk of damage to the PCB.Remove the low-melting point solder with de-soldering braid. Then you can solder in the new parts.Regards,Jay_Diddy_B

author: tooki
author_rank: Super Contributor
timestamp: August 11, 2024, 09:36:42 pm
content: Why bother with desoldering braid when OP has a top quality vacuum desoldering station?

author: unseenninja
author_rank: Regular Contributor
timestamp: August 12, 2024, 05:53:22 pm
content: Yeah, I don't even see a ground plane to make life harder.De-soldering something like that display is easy! Just give each pin some new fresh solder and then de-solder them. It will work fine with a good quality tool like your Pace. Don't forget to wiggle the Pace a little as you suck up the solder. All that's needed afterwards, if there is any sticking, is to just grab each pin with some fine nosed pliers and give them a wiggle. That will separate the pin from the side of the plated trough hole and when you wiggle the last stuck pin, the display will simply fall out.When you solder in the new displays, do better than the last person and clean the flux off properly afterwards.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: August 13, 2024, 06:50:02 am
content: Thanks, UnseenNinja. I have had the Pace for decades so am quite familiar with component removal, but was worried as I suspected the holes in the pcb would be very tight on the pins. Experience is that one, two or three pins are relatively easy to keep heated with a single wide chisel tip and remove the component but multiple pins are more difficult, but if the pin is tight in the hole there is no chance of clearance around the pin even if the solder is removed with vac.The faulty control and display board is back in the equipment for now until I modify the brightness of the displays on the spare board before installing it. I didn't want any more display failures as they are not cheap. I think the displays are from Kingbright (LSD582AEGR-20 dual common anode Red/Yellow) but can only find SBA56-21EGWA Red/Green. Not at all important as I am swapping all of them.I intended to reduce the 16v supply to the anodes via two red/yellow drive switches (3-transistor flip flops driven from the Atmega,) to lower the drive current to segments but due to the pcb layout it is very difficult to isolate that supply. I will have a try but may have to resign myself to changing the 57 1206 1k resistors hence the Talon query in 'Repairs'.Thanks all, BT

author: unseenninja
author_rank: Regular Contributor
timestamp: August 13, 2024, 07:03:47 am
content: If the pins are really tight and you can't clear the solder, the best alternative is some nice thick copper wire. Wrap it around the pins, flood everything with solder. The wire distributes the heat to all pins and you can get the solder liquid on every pin all at the same time.Good luck with isolating that 16V supply. I really wouldn't enjoy having to replace all those SMD resistors.