removing surface mounted chips



#2

On the HP 48G there are artices on how to upgrade the memory from 32K to 128K. I was able to open the calculator with not that much problem. Yet removing the existing memory chip was a failture.

My first attempt was as the auther suggested, to heat up each memory chip pin and bend them back one by one with a sewing pin. The problem with that is the gold foil circuit board connectors can get damaged. For me that was 3 foil contacts broken and the board is pretty much trashed.

How about using a dremel drill with the graphite cutting attachment to slice the IC off near the point of pin attachment, then desolder, then remove the individual pins one by one?

Does that sound like a good plan?


#3

Buying a HP48G+ seems a preferable (less risk) thing to do, you may sell your used 48G to recover some money, keep it for experiments, etc.

Of course, I appreciate the thrill and pride satisfaction that come with a successful "not standard" upgrade or repair, winning the challenge, etc.; but surface-mounted chips are a little beyond hobby level, and the 48G+ is readily available.

Oh, the increment in units sold may help HP to look at RPN as a not-so bad thing ... (wishful thinking)

#4

I've increased memory on two 48G's, and used the heat & bend one-by-one method -- it worked fine.

I used a small soldering iron with a sharp point. After the chip was off, I used solder braid (copper braid that soaks up solder when heated) to clean off the PCB pads.

I wonder if you're working too fast, bending the leads before the solder is melted? Or maybe a too-large iron, damaging the pads?

Refining your technique might be too expensive a process! Consider buying a 48G+. (Do you want to spend your time calculating with it or learn to be an electroncs repair technician?)

Good luck!

#5

I've removed many surface mount chips without unwanted damage. And I don't have a proper hot-air rework station either.
There are 2 techniques that I use. If, as here, I don't care about the old chip, then I pass a length of fine wire (5A fuse wire is ideal if you can still get it in the states) under one row of pins. Solder one end of the wire to a convenient (large) pad on the board.
Unse a temperature-controller iron with a fine bit to melt the solder on the pin nearest the other end of the wire. When the solder is molten (and not before!), gently pull on the wire to lift
the pin off the board. Then do the next pin, and so on. When one side of the chip is free, bend the chip package back and forth to break it off the other row of pins. Then desolder the remains of those pins one at a time. Use desolder braid (the finest you can find) to clean up the pads.
If I do care about the chip, then I use solder braid to remove as much solder as possible, and then carefully free each pin with a hot soldering iron and a (very) small screwdriver (<1mm tip width). I've done that on a couple of Voyager machines, and removed the
Nut CPUs without damaging either the chip or the PCB. But in general it's safer to damae the old chip if you don't care about it.
You should avoid applying any force to the pads on the PCB when they are hot. Heating them (e.g. with a soldering iron) reduces the bond strength between the copper track and the PCB. Trying to lift a pin before the solder has melted is likely to pull the track off the board.


#6

Excessive heat will also lift tracks.

It's a fine and delicate balance. And one that gets finer and more delicate as the pads and tracks get smaller.

My best advice is to get some old electronic equipment with surface mount components and practice and practice and practice until you have the technique faultless before starting on something that you don't want to kill.

I don't even try removing surface mount components. I have a friend who can remove them and reuse them (things like square chips with 20+ leads on each side) without damaging either the component or the board. And he does it with soldering iron and wick. Everyone who knows him agrees that he's a freak.


#7

Yes, Excessive heat is likely to damage the PCB as well (heck, it can also damage the chip).
I sometimes forget that not everybody uses a temperature-controlled soldering iron -- I regard one as _essential_ for electronic repair and construction. I'd not even attempt to work on an HP calculator using
a normal soldering iron.
Incidentally, I routinely remove and replace surface mount chips using a normal temperature-controlled iron and solder wick. Are you calling me a freak too?


#8

You're on thin ice here, Tony! ;-)


#9

Which bit? Whether or not I'm a freak?
I am not disputing that I am rather eccentric. I am just asking if I am being called a freak (whether or not it happens to be an accurate name for me doesn't matter at this stage :-)).
Incidentally, 'thin ice' is something else I'd not want to get too hot if I was on it :-)

#10

Occasionally on the worlds greatest junkpile (EBAY) you can find some really cheap stuff. I bought a really good vacuum desoldering rig for 40 bucks. I also bought an industrial strength hot air gun for 10 bucks. I have used it to remove and replace several HP41CV CPU chips. Shinks heat shrink real good. Just don't use it on your hair... remember it melts solder.

The trick to resoldering them is to make sure all the pins are straighened and square to the board. Use a very small tipped low wattage soldering iron (I really like the Antex units) and lots of liquid flux. The flux causes the solder to unbridge from between the pins (a tech at work does 288 pin packages this way... he solders the pins first, ignoring any shorts, drops on the flux, and then strokes down the pins to clear any shorts). Inspect your work with a 10+ power lens.


#11

Actually, resoldering these chips is a lot easier than removing them.
I use the same soldering iron (a 50W temperture-controller Weller) with a fine tip and some 28swg (thin) solder.
Make sure the pins are straight and position the chip onthe PCB. Makes sure it's the right way round. Tack down 2 opposite corner pins (just touch them with a well-tinned iron). Check again that the chip is the right way round.
Then solder the other pins, working along each side of the chip. Don't worry about short circuits at this stage. Then use some solder wick (desoldering braid) to remove excess solder and eliminate short circuits. Resolder the 2 pins you just tacked down, and clean those up with the solder wick.
That's it. It probably takes longer for me to type it up than to do it.

#12

Don't use a drill attachement to grind the legs as you will end up with fine metalic (copper and solder) dust all over the PCB etc. this is almost guaranteed to cause short circuits, especially if you then go around with a soldering iron.

Also if the drill slips you may well damage unrepairable section of the board / calc.

On mass produced equipment manufacturers usually junk the whole board if it doesn't work as re-work is expensive and not guaranteed to cure the problem. Most commercial returned equipment still under guarantee just gets whole boards swapped or sometimes the whole unit (e.g. HP with calculators!)

PS - It's still worth doing as a hobbyist but not for companies who are simply interested in $$$.


#13

No, don't use any form of cutting/grinding tool. Not just for the reasons already given (it'll spread conductive dust everywhere and if it slips it'll make a right mess!), but also
it'll apply a sideways force to the pins/PCB pads which is the best way I know to rip the pads off the board.
As regards 'professional' SMD rework, it may not make sense in production, but every research / development
lab I've worked in does a fair amount of hand removal and replacement of SMD parts on prototype boards. It's sometimes the only way to debug a design. And of course it makes sense for a hobbyist. And it's the only way to repair
some older HP calculators (you take a chip from one broken machine and put it into another, thus getting one good machine from 2 dead ones), so it's essential to be able to do this if you want to do HP repairs.


#14

Actually hand mounting surface mount chips goes on quite a bit in production environments. A company I worked for had a board assembly operation in Mexico and over a years time we hand mounted over a million large (132+ pin) surface mount chips. Our "Manuel inserters" produced a higher yield of working boards and lower chip damage rates than automatic equipment could at the time.

Now most large chips are in ball grid array packages that cannot be hand soldered because there are no pins to solder... there are solder balls on the back side of the chips that IR ovens melt... once mounted, these things must be inspected by X-ray machines. Rework is a real problem and a single defective joint can cause an entire board to be scrapped.


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