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How to replace bad CPU on circuit board?

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elektroda.net NewsGroups Forum Index - Repair Electronics - How to replace bad CPU on circuit board?

Robbie Hatley
Guest

Thu Mar 05, 2020 2:45 am   



At my workplace, I'm often faced with circuit boards which WOULD be
repairable, if not for the fact that the CPU on the board is fried.
(These CPUs are not like the CPUs in PCs; they're 15mm-square
$5 8-bit 8MHz 100-pin CPUs with 25 pins on each side, surface-mount
soldered directly to circuit boards.)

Repairing these boards is maybe impossible, but I'm investigating
options. The 2 main problems that would have to be solved are:

1. Remove CPU from board. (CPU doesn't need to survive, but board does.)
Heat gun? (Might damage surrounding components.)
Solder bath? (Might unsolder surrounding components.)
Cut pins? (Might damage traces.)
OTHER???

2. Extract software from good CPU???
(The board maker won't supply the software due to copyright issues.
The boards DO have programming headers, presumably for use with an
external programming module which connects to a PC via USB; but I
don't know if such programmers can work in reverse, READING software
from a CPU instead of WRITING to it.)


Anyone here have some ideas regarding these two issues?


--
Robbie Hatley
Hatley dot Software at gmail dot com

jfeng@my-deja.com
Guest

Thu Mar 05, 2020 3:45 am   



I suggest investigating ChipQuik. Do a Google search, and look at the videos. It looks like you can get it from Digikey, Mouser, etc.

Disclaimer: I have not used it myself, but was amazed when I saw it demonstrated.

John Robertson
Guest

Thu Mar 05, 2020 3:45 am   



On 2020/03/04 5:52 p.m., jfeng_at_my-deja.com wrote:
Quote:
I suggest investigating ChipQuik. Do a Google search, and look at the videos. It looks like you can get it from Digikey, Mouser, etc.

Disclaimer: I have not used it myself, but was amazed when I saw it demonstrated.



ChipQuik works as advertised, however I think you might further ahead by
investing in a hot air or Infra-Red/Hot Lamp service system. These start
fairly cheaply and, depending on what you board looks like, can be
pretty easy to run.

If only a few boards, then ChipQuik is probably best, otherwise hot air
or IR is the way to go for chip replacement.

The question is - how many boards are we talking about and what is the
replacement cost? A good hot air system will cost a few hundred dollars
but can simplify the chip replacement process considerably.

I'd start by watching some youtube demonstrations of "hot air chip
replacement" to get an idea of how it is done.

John :-#)#

Michael Terrell
Guest

Thu Mar 05, 2020 4:45 am   



On Wednesday, March 4, 2020 at 8:31:42 PM UTC-5, Robbie Hatley wrote:
Quote:
At my workplace, I'm often faced with circuit boards which WOULD be
repairable, if not for the fact that the CPU on the board is fried.
(These CPUs are not like the CPUs in PCs; they're 15mm-square
$5 8-bit 8MHz 100-pin CPUs with 25 pins on each side, surface-mount
soldered directly to circuit boards.)

Repairing these boards is maybe impossible, but I'm investigating
options. The 2 main problems that would have to be solved are:

1. Remove CPU from board. (CPU doesn't need to survive, but board does.)
Heat gun? (Might damage surrounding components.)
Solder bath? (Might unsolder surrounding components.)
Cut pins? (Might damage traces.)
OTHER???

2. Extract software from good CPU???
(The board maker won't supply the software due to copyright issues.
The boards DO have programming headers, presumably for use with an
external programming module which connects to a PC via USB; but I
don't know if such programmers can work in reverse, READING software
from a CPU instead of WRITING to it.)


Anyone here have some ideas regarding these two issues?


I used to replace Motorola MC68340 processors fairly often. I used solder wick to remove most of the solder, then an Ungar Loner soldering iron with a .015" tip, along with a tiny curved pick. I would apply a slight pressure to a pin, as I touched it with the soldering iron tip. It would op free, and the pic would remove enough heat that it stayed free from the pad. It took really good soldering skills but both the board and the IC were undamaged. These were 288 pin, 72 per edge. The programming port was JTAG, not USB.

As far as reading the firmtware, you didn't give a part number. Some have a bit that can be set to stop you from reading out the firmware.

Ralph Mowery
Guest

Thu Mar 05, 2020 5:45 am   



In article <r3pkpr$fk2$1_at_dont-email.me>, Hatley.Software_at_gmail.com
says...
Quote:

Repairing these boards is maybe impossible, but I'm investigating
options. The 2 main problems that would have to be solved are:

1. Remove CPU from board. (CPU doesn't need to survive, but board does.)
Heat gun? (Might damage surrounding components.)
Solder bath? (Might unsolder surrounding components.)
Cut pins? (Might damage traces.)
OTHER???



Can't help with the programming.

To remove the chip, a hot air gun is the way to go. There are 2 ways to
help keep the air where it is wanted. Special tips that are just large
enough to blow the hot air on the pins. Use some kapton tape over areas
where you do not want the hot air. It is usually like an amber colored
celophane tape and is heat resistant.

It depends on your budget and how many boards you plan on working on.
There is some hot air stations on ebay for around $ 60 that may get the
job done. There are better ones that are a couple of hundred bucks.
Having a good stereo microscope helps. An Amscope se400z scope with 10x
lenses is very good for the price at around $ 200.

There are plenty of youtube videos that show how to do it. Just get
some old computer boards with smd on them and practice. Plenty of flux
is one of the better tips.

Plan on spending around $ 350 to $ 400 to get the minimum equipment
needed to do a decent job.
The microscope and hot air station is the most.

Rob
Guest

Thu Mar 05, 2020 3:45 pm   



Robbie Hatley <Hatley.Software_at_gmail.com> wrote:
Quote:
At my workplace, I'm often faced with circuit boards which WOULD be
repairable, if not for the fact that the CPU on the board is fried.
(These CPUs are not like the CPUs in PCs; they're 15mm-square
$5 8-bit 8MHz 100-pin CPUs with 25 pins on each side, surface-mount
soldered directly to circuit boards.)

Repairing these boards is maybe impossible, but I'm investigating
options. The 2 main problems that would have to be solved are:

1. Remove CPU from board. (CPU doesn't need to survive, but board does.)
Heat gun? (Might damage surrounding components.)
Solder bath? (Might unsolder surrounding components.)
Cut pins? (Might damage traces.)
OTHER???

2. Extract software from good CPU???
(The board maker won't supply the software due to copyright issues.
The boards DO have programming headers, presumably for use with an
external programming module which connects to a PC via USB; but I
don't know if such programmers can work in reverse, READING software
from a CPU instead of WRITING to it.)


Anyone here have some ideas regarding these two issues?


That kind of chips is actually EASY to handle, compared to the newer
style BGA where you only see a square chip directly mounted on the board
and no pins whatsoever. See the other replies.

The problem will be the programming, you need to investigate that first.
Programmable CPUs usually come with a readback prevention and when that
is activated (it is an option during programming) it will be difficult
or impossible to get the software, and then the whole replacement thing
is not a topic anymore.

What is the part number of the CPUs?

Jon Elson
Guest

Thu Mar 05, 2020 11:45 pm   



On Wed, 04 Mar 2020 17:31:34 -0800, Robbie Hatley wrote:

Quote:
1. Remove CPU from board. (CPU doesn't need to survive, but board does.)
Heat gun? (Might damage surrounding components.)
Solder bath? (Might unsolder surrounding components.)
Cut pins? (Might damage traces.)
OTHER???

Best is a hot air desoldering tool with a square nozzle to heat the leads
and board right next to leads. These tools are amazing.

If not, you can use an X-acto knife to score the pins right at the body
until they break. If careful, it does not damage the board. Cut the
leads on 3 sides completely, then score the leads on the 4th side part-
way and bend the IC a few times and they break off. Then, sweep the
leads off the pads with a soldering iron. Clean up remaining solder with
braid, and replace chip.

But, the hot air tool is 100X better!
Quote:

2. Extract software from good CPU???
(The board maker won't supply the software due to copyright issues.
The boards DO have programming headers, presumably for use with an
external programming module which connects to a PC via USB; but I
don't know if such programmers can work in reverse, READING software
from a CPU instead of WRITING to it.)

Some chips can be read out, some are protected and you CAN'T read the
contents.

Jon

Jeff Liebermann
Guest

Sat Mar 07, 2020 8:45 am   



On Wed, 4 Mar 2020 17:31:34 -0800, Robbie Hatley
<Hatley.Software_at_gmail.com> wrote:

Quote:
At my workplace, I'm often faced with circuit boards which WOULD be
repairable, if not for the fact that the CPU on the board is fried.
(These CPUs are not like the CPUs in PCs; they're 15mm-square
$5 8-bit 8MHz 100-pin CPUs with 25 pins on each side, surface-mount
soldered directly to circuit boards.)


I can't tell what type of package that might be. My best guess(tm) is
one of these:
<https://en.wikipedia.org/wiki/Quad_Flat_Package#TQFP>
A maker and part number for the CPU would be helpful.

Quote:
Repairing these boards is maybe impossible, but I'm investigating
options. The 2 main problems that would have to be solved are:

1. Remove CPU from board. (CPU doesn't need to survive, but board does.)
Heat gun? (Might damage surrounding components.)
Solder bath? (Might unsolder surrounding components.)
Cut pins? (Might damage traces.)
OTHER???


Hot air SMD desoldering station. Something like this:
<https://www.google.com/search?tbm=isch&q=hot+air+smd+desoldering+station>

You'll need a nozzle to match your chip:
<http://new.ldbenterprises.com/smd-rework-station-nozzles/>
You'll need to know the IC package type to order the proper nozzle.

There are videos on YouTube on how to use these:
<https://www.youtube.com/results?search_query=smd+qfp+desoldering>

You'll also need flux and flux remover. A good binocular microscope,
fan to blow away the smog, and an aluminum foil heat shield, are quite
helpful.

Quote:
2. Extract software from good CPU???
(The board maker won't supply the software due to copyright issues.
The boards DO have programming headers, presumably for use with an
external programming module which connects to a PC via USB; but I
don't know if such programmers can work in reverse, READING software
from a CPU instead of WRITING to it.)


Most uP firmware is copy protected. That may not be possible. You'll
need to get the instructions from the manufacturer of the product. If
the device has a JTAG port, you might be able to do something with
that.

--
Jeff Liebermann jeffl_at_cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

legg
Guest

Sat Mar 07, 2020 7:45 pm   



On Wed, 4 Mar 2020 17:31:34 -0800, Robbie Hatley
<Hatley.Software_at_gmail.com> wrote:

Quote:
At my workplace, I'm often faced with circuit boards which WOULD be
repairable, if not for the fact that the CPU on the board is fried.
(These CPUs are not like the CPUs in PCs; they're 15mm-square
$5 8-bit 8MHz 100-pin CPUs with 25 pins on each side, surface-mount
soldered directly to circuit boards.)

Repairing these boards is maybe impossible, but I'm investigating
options. The 2 main problems that would have to be solved are:

1. Remove CPU from board. (CPU doesn't need to survive, but board does.)
Heat gun? (Might damage surrounding components.)
Solder bath? (Might unsolder surrounding components.)
Cut pins? (Might damage traces.)
OTHER???

2. Extract software from good CPU???
(The board maker won't supply the software due to copyright issues.
The boards DO have programming headers, presumably for use with an
external programming module which connects to a PC via USB; but I
don't know if such programmers can work in reverse, READING software
from a CPU instead of WRITING to it.)


Anyone here have some ideas regarding these two issues?


You can desolder gull-wing leads, pin by pin, by slipping a
1-2mil nickel-iron strip between pin and pad, while heated.

The strip can be progressively advanced along the pins, as
each is freed. The nickel content prevents solder adhering
to the strip.

These strips are salvageable from old stick-on anti-theft
devices, often found on CD cases or small electronic device
packaging. You'll find 2x1mil and 1x2mil strips in each one.

These strips will fasten into a standard exacto knife handle,
and can be angle-cut with scissors, bent or restraightened
to your prefered shape. 2mill probably is easiest to start
with.

An angle cut works best to start from a package corner.

No point in removing a part that can't be sourced for
a replacement, including code.

RL

elektroda.net NewsGroups Forum Index - Repair Electronics - How to replace bad CPU on circuit board?

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