Rework soldered joint not shinny, but matte. - Page 1
Forum section: electronics / beginners

author: BlackICE
author_rank: Regular Contributor
timestamp: August 09, 2023, 07:01:08 am
content: Dumb me mounting an SMD diode backwards so I had to remove a part reverse it and resolder it. Since the pads and part already had plenty of solder I didn't add any more or used any flux. I just remelted what was there. I heated the joints up until the solder flowed and was shinny, but upon cooling, it didn't stay shinny but instead turn matte.I'm guessing I should have added some flux. Some of the solder must have oxidation that floated to the surface upon cooling. I originally used a flux core 67 Sn / 37 Pb solder. Is this what happened?

author: RoGeorge
author_rank: Super Contributor
timestamp: August 09, 2023, 11:22:45 am
content: If the original soldering was made with Pb free, and you didn't clean the pads, than the former Pb free mixed with the Sn/Pb solder, and the result may vary. Try reworking that solder joint with a little flux added.If not happy with the end result, remove the part, clean the remaining solder from both the pads and the diode, then solder again with Sn/Pb, and it should be shiny.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 09, 2023, 12:41:41 pm
content: Lead does very weird things with tin. Especially at quite low lead concentrations it can wreak havoc with your solder.It's like: 1% lead contamination in a lead free solder can lead to 80% strength reduction, and solder is not the strongest connection to begin with.In practice, this can happen if you have still a bit of leaded tin on your solder iron, and then put it in a lead-free solder joint.----------------But probably it's less alarming.What most likely happened is that you did not use flux. Flux is essential for soldering, and without it a solder joint does not form properly.The easiest way for beginners is to remove the old solder from the joint and apply new solder (All solid wire for electronics soldering contains flux). If you do this more, you can buy flux separately, in pen or syringe or other form. (the pens seem most popular for rework).You can also do a test yourself:1. Make a fresh solder joint.2. Put your iron back in the joint and wait 10 to 20s.3. All the flux has now boiled off, and you can't solder properly without adding new flux (from the solder).One of the tricks of learning proper soldering is to do it with the right speed.If you do it too fast, then the joints do not get hot enough and connections are not properly formed.If you do it too slow, then all the flux has evaporated (that smoke that comes off during soldering) and it also does not work anymore.

author: u666sa
author_rank: Frequent Contributor
timestamp: August 09, 2023, 02:41:18 pm
content: What brand solder you use and flux? That's very important.From good cheap brands, Mechanic solder and flux is best in my opinion.Mechanic M60 0.5mm tin60 lead40 Cl0.05% Expansion75%Machanic TY-V866 1.0mm tin63 lead37 Cl0.05% Expansion75%Mechanic flux MCN-UV10 plus there alloy flux pusherThese are el cheapo products, solder last you a while, flux ends quicker, so you get several tubes. Cheap is good because you tend to use cheap more often.The no name bad solder does do what you described, and the no name flux leaves crap on your board and almost impossible to wash. So I think you using bad brand solder.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: August 09, 2023, 03:28:05 pm
content: Quote from: BlackICE on August 09, 2023, 07:01:08 amThe easy way to remove SMD parts is with hot air, the low cost 858 costs around 30 GBP on eBay. Lift the component with curved tweezers. Using a soldering iron and solder wick, clean off the pads, then re-solder the component.SJ

author: BlackICE
author_rank: Regular Contributor
timestamp: August 10, 2023, 10:39:14 am
content: I think I know what caused it now.I added some flux and reheated the connections, still matte. So that eliminates contaminates. So my guess is because I never removed the original solder on the pads. Most likely non-leaded solder mixed of the new solder cause the mixture to be non-eutectic and upon cooling and contracting caused an ill-regular matte surface finish.BTW I was using Chipquik 63/37 2% RA solder. I don't have a hot air station so I used 2 soldering irons instead.

author: BillyO
author_rank: Super Contributor
timestamp: August 10, 2023, 04:31:23 pm
content: Quote from: BlackICE on August 10, 2023, 10:39:14 amThat's it. You need to clean the pads as well as you possibly can. Unlike a good stew, re-heated solder gets worse.

author: tooki
author_rank: Super Contributor
timestamp: August 11, 2023, 04:09:50 pm
content: Even when not mixing solder types, "overcooking" the solder will result in matte joints prone to icicles and the like. The longer it's heated, the more the solder oxidizes, and the more it dissolves the base metals of the component leads and PCB pads, producing an alloy of unknown composition. When a joint starts behaving badly, I will remove the solder from the joint, and I'll also "rinse" the iron's tip with fresh solder a few times to eliminate the overcooked solder. The joint usually behaves much better after doing this.

author: Shock
author_rank: Super Contributor
timestamp: August 12, 2023, 12:53:53 am
content: Just test solder on another scrap board with a few components and their solder pads cleaned, a good soldered joint looks the same as a good soldered pad. By adding fresh solder and wicking off a few times almost all traces of old solder will be removed. You can test your solder alloys bond by cutting into it with and old flatblade screwdriver corner or a dental pick. A well bonded joint will not release from the pad.

author: SL4P
author_rank: Super Contributor
timestamp: August 13, 2023, 10:16:08 am
content: Does the PCB have a circle printed withPbinside… crossed out ??