I have one bad capacitor in motherboard that needs to be replaced. I add some solder to connection so that heat transfers to solder joint and then i pull out bad cap easy. But i notice that on some motherboards, or device that i try to remove, the solder that i add will not mix with solder that is used in production, so the solder i add melts but "factory" solder stay hard and i cant make it melt. Any idea what is wrong or what setup should i use ? Also the heat on solder station was 400C, and i try two solder stations with different tips. I Also try the heat gun and end up damaging pcb on one spot.

 

Solder has different melting points which as based on the metals used. You are most likely running into new unleaded variety. You will need a higher temperature to desolder them. I don’t like the new solder either.

 

Some houses are building boards with high temp solders. They are also moving away from leaded solders because of environmental concerns.

Reading your comments, I think I understand that you've used your "regular" solder to bring a larger thermal mass to a joint and remove a cap. That's pretty much how you solder, bringing thermal energy to the joint. A clean iron will seldom melt solder on a board without the introduction of a flux of some sort. And there are a variety of types of flux available. Flux is not my area of expertise. Also, from your comments it sounds like the cap you removed has nothing to do with the board you're trying to work on, the mother board.

In the case of the mother board, I don't think they've used high temp solder. HT solder is typically used in areas that see excess heat and current. Standard tin/lead solder will degrade and melt under those circumstances. Your mother board should not be seeing those kinds of temperatures or currents. Rather, I suspect the MB has been built using "Lead Free" solder. Largely they are silver solder and require a much higher temperature to work with. 400˚C is equal to 752˚F. My Radio Shack solder station is capable of 800˚F and can barely manage to melt LF solder. Given the possibility that your joint may be a power or ground plane, that will act like a heat sink and pull heat away from the joint. You may find it nearly impossible to melt solder under that condition. Even bringing a molten blob of solder to the joint, the heat will dissipate quickly.

Just recently I did some work for a defense contracted board assembly house. Some of their boards have such large ground or power planes that you had to put the board over a board heater and bring the temperature up across the entire board. That way the planes don't suck away the heat. Without that the solder will not flow through the entire joint. Surface Mount Devices (SMD) are easier, but they are also typically smaller joints. Even still, there are times when use of an 800 degree tip is required. However, the most common heat used was a 700 degree tip.

To make solder mix you have to bring sufficient thermal energy (heat) to the joint. That's risky because you can lift pads and traces. However, melting solder isn't just about pasting a conductive material over a joint; it's more about creating a molecular bond between metallic surfaces. When solder (regular solder) melts and makes a joint it molecularly bonds with the copper lead and the pad. Or if the lead is pre-tinned, since tin is one of the elements in regular solder, the tin bonds. Steel leaded components also bond fairly easily to tin/lead. Gold plated leads (plated to prevent oxidation and NOT for better conduction) tends to make solder joints brittle. It's called "Gold Embrittlement". Standard manufacturing practices sometimes require solder to be melted into the gold plated joint then wicked away before making a final solder joint.

Sometimes when I scrap a board I use a paint stripper heat gun to melt en-mass the solder joints. I then shake the parts off the board with a violent shake. Then I have to sort through the parts I want. The rest, things like the SMD caps - without a capacitance meter you don't know their value. And even knowing their farad rating you still are unsure of the voltage. PLUS, that high thermal shock can crack those caps. Especially ceramic caps. But when scrapping a board I realize there's going to be - um - scrap. More often than not I'll pick a part I want and then go after it in a more conventional manor. LF solder is difficult to work with, but not impossible. But it requires a lot of heat.

The reason why you're seeing difficulty with some boards is likely due to being Lead Free solder. LIKELY. Not specifically. There can be other factors at play. Flux can help.

 

that's a great point @Tonyr1084 ever try to solder a 0 AWG wire to lug? It takes a lot of heat and much more than a tiny soldering iron can provide...

 

@Tonyr1084 Thank's you explained it all.

 

First, in many soldering stations those T on the screen very poor corresponds to the reality. My station shows 400 when iron tip have (measured by IR) 270, thus its not a criterion at all. Second, most probably there is used a POS60 (270C) or POS40(350C) whilst nowadays "lead-free" may melt already at 180-230C. Third, probably outer layer is lacked or/and oxidized, thus the thermal contact is nil. Just little bit scratch it beforehand. And last - probably soldering station hammer is si tiny that mass is too small for oldy-goody days soldering made by 6-8 mm thick massive copper bar having the real mass.