Question About Desoldering Large PCBs

Thread Starter

BlackMelon

Joined Mar 19, 2015
93
1596037478399.png

Dear All,

I am trying to repair an inverter welder. I am trying to remove some components at the top of this picture. However, as I try to desolder those components, I think the heat applied to a pin has spread out all over the PCB's traces. As a consequence, it took very long to heat the solders up to the melting point to suck them out. I would like to know what is the better way to solve this issue?

Sincerely,
BlackMelon
 

jpanhalt

Joined Jan 18, 2008
10,227
Use flush side cutters to cut the leads. Remove the IC's body, then remove each leg by itself. A lot less heat is needed. You can do the same with SMD's that have legs. Alternatively, hot air works. You do not need a fancy, temperature controlled hot-air rework station.
 

Thread Starter

BlackMelon

Joined Mar 19, 2015
93
My desoldering gun works great on regular PCB's but on very thick metal areas such as copper rivets, I help it along with my Weller 100/140 watt soldering gun.
Thank you. I will look for that on online shops.
Use flush side cutters to cut the leads. Remove the IC's body, then remove each leg by itself. A lot less heat is needed. You can do the same with SMD's that have legs. Alternatively, hot air works. You do not need a fancy, temperature controlled hot-air rework station.
That's sounds rough to the IC. However, if it can't help, I will do as you suggest. Thank you.
What is the tip style, temperature and wattage for the iron?
The tip is iron. Wattage is 40W. No temperature control available. Heating element is nichrome. Please refer to the following link.
http://www.hakko.com/english/products/hakko_red_tips.html#productNav
 

SamR

Joined Mar 19, 2019
2,574
Put flux on what you want to remove. It will help if there is any oxidation of the old solder which can act as an insulator layer. When iron is hot, clean tip and wet it with a tiny bit of solder. That will help with the heat transfer. If the iron has a replaceable tip, put a big one on. Once again, the trick is heat transfer fast. Cutting the to-be removed part down is excellent advice to remove thermal mass which sucks up the heat being transferred, but make sure there is enough left to grab onto with some needle-nose pliers. Kinda tricky but use the pliers in one hand to put light tension on the piece to be removed while applying heat with the iron in the other hand on the solder. Go at it from both sides of the board, pliers on the part side, and iron on the solder side. Not always easy to do and take a bit of coordination. Good Luck and let us know how it went.
 

jpanhalt

Joined Jan 18, 2008
10,227
Rather than pliers/forceps to apply tension, I will often slip a thin blade (Xacto or tip of jewelers screwdriver) under the device. The control is better.
 

SamR

Joined Mar 19, 2019
2,574
I actually have a soldering tool with a slotted tip like a mini prybar that I use often for intact devices. Even with the vacuum desoldering gun there will often be parts that need a little help getting free.

IMG_0694[1].JPG
 

Thread Starter

BlackMelon

Joined Mar 19, 2015
93
Put flux on what you want to remove. It will help if there is any oxidation of the old solder which can act as an insulator layer. When iron is hot, clean tip and wet it with a tiny bit of solder. That will help with the heat transfer. If the iron has a replaceable tip, put a big one on. Once again, the trick is heat transfer fast. Cutting the to-be removed part down is excellent advice to remove thermal mass which sucks up the heat being transferred, but make sure there is enough left to grab onto with some needle-nose pliers. Kinda tricky but use the pliers in one hand to put light tension on the piece to be removed while applying heat with the iron in the other hand on the solder. Go at it from both sides of the board, pliers on the part side, and iron on the solder side. Not always easy to do and take a bit of coordination. Good Luck and let us know how it went.
Thank you very much. This is very helpful and super informative! I appreciate your advice.

Also, thank you very much jpanhalt.
 

dl324

Joined Mar 30, 2015
11,518
It's PTAA. So should I try changing the tip to PTE?
It depends. The dimensions of a tip should match the components being soldered. Having a tip that's too large will cause different problems; too much heat can cause damage.

I've used a PTA type tip to solder pieces of copper clad together to make boxes. The trick is to apply enough heat to get the solder melted.
clipimage.jpg
Conventional wisdom would indicate that the tip I used was too small... I didn't even clean the copper or use additional flux.
 

SamR

Joined Mar 19, 2019
2,574
Here are the tips I usually use. Big chisel for removal, smaller chisel for most soldering, and the small round point for delicate work.

1596130475036.png
 

jpanhalt

Joined Jan 18, 2008
10,227
Very few components are attached with a single lead. Wires are the principal exception. I resisted getting a hot air gun for years and depended on different tips or my trusty 80 W with a big tip when desperate. Sometimes, as a last resort, I would use a propane/butane torch.

I broke down about 2 years ago and got a hot air gun (Wagner HT400). It does a neat job and I have even used it to solder some large, fine pitch SMD chips. It is not temperature controlled, so you have to be careful, but it is also cheap. Mine is the older straight version.
 

SamR

Joined Mar 19, 2019
2,574
even used it to solder some large, fine pitch SMD chips
How did you keep it from blowing away? I've been looking for a nice heavy pair of angle nose tweezers for that. Yep hot air is the ticket for SMD removal although I buckled up the cutting mat I use to protect the benchtop with from the heat even with a silicone rubber mat under the ckt board. I like the vacuum pump for the through-hole stuff but it's not perfect and a pain in the tuchus to clean the sucked up solder out of and the filters for it aren't cheap.
 

jpanhalt

Joined Jan 18, 2008
10,227
The velocity is not that high. Controlled with height. It was a relatively large, 64-pin leaded chip (TQFP). That cheap hot air gun is really not for re-flow soldering, but it can work. If I were doing a lot of hot-air reflow, I would definitely get something more designed for that.

Some people use Kapton tape to protect other stuff. I just use aluminum foil and intentionally crinkle it before use. If I need to hold an edge down (rarely), I use Kapton, but I don't view Kapton as a very good heat insulator. Aluminum foil is great. NASA uses gold foil (maybe its just gold plated). I have other uses for gold (today's spot $1943.50; quite a change from the $250 or so when Soros made his run at the Bank of England)

I rarely use tweezers/forceps to hold anything down. Yes, I use them to place parts, but once on the board, I use slightly pointed and rounded pieces of 0.032 music wire (smaller is OK). It is springy and gives nice feedback for how much pressure you are applying.

My bench is an old Formica covered computer table. I often use pieces of wood (usually birch, maple, or ash) to work on. The species doesn't matter much. Birch,maple, and ash are not very resinous and give great heat insulation. Balsa will also work and is probably an even better insulator.
 
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SamR

Joined Mar 19, 2019
2,574
I'm planning on trying a piece of 3/4" plywood next time I go crazy on a scrap PCB removing surface mount parts. I'll find out how it does and if it delaminates from the heat. My benchtop is actually a piece of Formica countertop with the front rolled edge. Cutting mat on that and then the small 10"x14" silicone rubber ESD mat which is easier to clean in the sink.

On SMDs I usually tweak it into place and stick a finger on it while I tack one leg down quickly with my iron before I burn my finger. That and lots of paste flux and then it pretty much won't go anywhere. Never tried the hot air to do it. Old Shakey Hands don't like SMD. I know about the Kapton and have some but never used it for heat shielding while soldering. I do have some different sized all copper alligator and flat jawed clips I use for heat sinking on device legs sometimes if needed.
 

jpanhalt

Joined Jan 18, 2008
10,227
For cutting stuff, I use a "self-healing" mat from JoAnn Fabrics or other craft stores. It's a typical green mate marked with dimensions. It doesn't heal completely, but mine are 25+ years old and still usable. ( I took a large mat and cut it into usable sections -- small rectangles to a single piece that is 48" x about 10".)

Try the wire, you will like it. Applies adjustable pressure and no burnt fingers. I also cheat with a little medium or thick CA from time to time. Put a puddle on bare copper. It won't set off. Then use those needles to transfer a tiny dot (<0.1 uL) to where you want it. Apply pressure and that helps set it off, or just wait.
 
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