Surface mount component rework/modification

[walter]

How do you rework fine surface mount components? I recently reworked some SOT-563 components where the pcb pads 4 and 6 were reversed on the software. I had to lift the components, isolate pads 4 and 6, re-mount the component, and run a wire from pin 4 to pad 6, and pin 6 to pad 4. The SOT-563 is a six pin IC measuring 1.5mm x 1.1mm, the pins are .2mm.  We have stranded wire here, so I used one strand from that. My soldering iron tip is .2mm. The boards that I reworked were lead free, so I reflowed with leaded solder. I applied flux with a syringe, but the syringes we have are not the smallest, so I usually flooded the area. I used kapton tape to hold the wire strand in place. This process was very difficult and time consuming.
The next batch of boards, the board house did the rework. They stood the ICs up on pins 1,2,3, and ran mag wire to pads 4,5,6. Unfortunately, some of the ICs are in an RF can and shorted when the lid was placed, so I reworked those.
I have a Metcal solder station, and a Hakko solder station. The Hakko also has hot air, although I don’t have much experience using the hot air. Is there anybody here that does this kind of work with any opinions on hot air vs. solder tips?

 

[Rochey]

hot air all the way with SMD... that and buckets of flux.

the traces are so small with SMD that using a soldering iron and any "force" can really mess them up.

 

[JohnRoberts]

Hot air is superior for standard SMD rework, but if adding point to point wires you have little alternative to using an iron. The 6 pin SOT is still relatively wide pin spacing, smaller finer pitch packages are a biotch.. I had one small class D audio amp chip with a heat sink on the bottom i couldn't even prototype using old school point to point.

JR

 

[bruce0]

The hot air is great.  You need a little set of long handled antistatic tweezers which really help.  Kapton tape helps to protect other components.  Preheating the board helps (I suspend it in clips and go at it a bit with a heat gun, not as precise as i would like).  If you pump the hot air too fast it really does blow components off the board, it doesn't take too much force.

There is liquid flux (not a paste) for this work and it is much better.

There is stuff called "chip-quick" which can help you remove a component without hot air, it is a low melting point solder that lowers to melting point of all the solder it mixes with (like 70 degrees c) but you must clean it all off before resoldering.

Some IC's are glued down I have had trouble with that.

There are lots of good youtube video's on this, it helps to see it to get the hang of it.  For instance the small multipin parts position themselves with surface tension of the liquid solder but until you see it it is hard to imagine.

My process is

(to take it off)
Shield the innocent with Kapton
preheat board from bottom
heat component to remove
remove with tweezers

(fix replace modify repair then
to put it on)

board was hot, when cool clean with flux cleaner/alcohol
put flux on board where parts go
touch the flux with solder on an iron to put a small hump of solder on each pad (there may be a better material to do this with)
if a pad has too big a hum remove it with solderwick or a clean iron tip, careful you don't remove the pad)
Clean AGAIN with flux cleaner alcohol
put flux on board where parts go
(sometimes with reused parts I dip the  pins in excess  flux on the board)
put parts on top of flux approximately where they go
preheat board from bottom
heat part with air (not so much as you move the part)
when the solder is molton tap part into place with tweezers (if it hasn't done it itself) or tap it on top to make sure all pins are on the surface (in case there is a flux bubble etc under the part)

It cools off.

Problem areas are Very heavy parts (surface mount inductors) that take so long to heat that I worry about damaging other stuff.  With those I use an iron to heat the part contacts best I can to heat it first for removal, and I preheat the part with hot air before putting it back on the board.

I haven't tried to add little wires, but I would imagine you could wind it around a leg, or bend a leg (if it is not QFN) up for contact access.

 

[walter]

I did try to bend the leads up, but the case just fractured, so that is not an option. I have had problems soldering resistors and caps when I hold the component with tweezers and solder one side down, release the tweezers, solder the other end but the heat goes through the component and the component is on the tip of the iron instead of on the pcb. As I work on smaller components, 0402, 0201, this does not seem to happen.
I prefer liquid flux, we need solder paste, for now I use .010 solder, but that is problematic, it goes up the iron or inconsistent feed. I also use #1 solder wick, but that is too big and gets stuck to adjacent pads. I’m working with the smallest tools, and they are too big for the smallest components.
We have an ERSA  IR rework station and a Pace Sodr Tek hot air rework station. I have seen the Pace used once since I’ve been here. I set them both up a few months ago to see if they worked and they do. But without using them regularly, they are not practical for one off jobs. Management is getting a new rework station installed, and I imagine there will be some training included. I don’t know if it will be useful for mods. They want to be able to replace BGA components in house. My Hakko does have an air pressure adjust so that helps keep the component from blowing away. I’ll try to use it more.