Author Topic: CPLD to replace 74xx chips in an old circuit  (Read 4500 times)

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Offline legacy

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #50 on: July 29, 2019, 03:00:27 pm »
You just would pay upfront 3x the price for boat anchor which probably would fall apart even before delivered.

You have a problem, and you are looking for a solution.

Before shipping to that company, I asked a lot of guys, colleagues, and friends. Most of them answered that "replacing an SMD chip is a piece of cake", like if it was the canonical sentence to say, but when they saw the motherboards and understood what needed to be done, well at the end of the month, nobody did actually did the job.

I waited a second month, and again I only collected funny answers like "oh, sorry, I am so busy with the new vegetable garden, this year I want to grow onions", "oh, sorry I have to go the dancing club with my wife otherwise she will ask for divorce", etc, and yet again nobody wanted to do the job. I even offered money, and again no one accepted the call.

In the end, two months later, I had two choices: to trash everything and forget it, or to send an email to the company and to be prepared to pay a very salt bill  :-//
 

Offline legacy

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #51 on: July 29, 2019, 03:04:03 pm »
Using sockets ensures poor reliability.

I made my first SBC in 1992. It's a simple 8051 board with each chip (CPU, RAM, ROM, buffers, latch, etc) on a DIP socket, but not the tulip-kind, the other kind. We are in 2019, so, say ... 27 years later it's still working! Oh, and the ROM has been removed and reinserted a lot of times because my first ROM emulator arrived only 8 years ago, and before it, I had to remove the UV-ROM, erase it with a UV-lamp eraser, reprogram it, and reinstall it.

I don't know the exact number, but for sure the ROM socket has seen a thousand re-insertions!
 
So, I think you were talking about tulip DIP sockets, which, in my opinion, are poor reliability simply because everything I have tried .. has only allowed 50 re-insertions before failing.
 

Offline SiliconWizard

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #52 on: July 29, 2019, 03:14:53 pm »
Yeah. Desoldering TH chips through 8 layers or more (like on motherboards) was often a nightmare, so...

DIP chips on DIP sockets, you do not have do desolder them.

Uh huh. Well, maybe all the boards you design with TH chips have a socket for ALL chips (not going to discuss reliability as it has already been discussed), but it's not the case for most boards produced at a reasonable volume. The added cost of the sockets + production cost of inserting the ICs in them is not worth it.

So yeah, that may be a solution for your very own developments, but not a general truth. I have never seen a commercial motherboard, even from over 30 years ago, that had sockets for all ICs...

So you're basically talking about your own needs for repairing your own boards. I have had to repair a few PC motherboards in the past with a few TH chips (such as BIOS Flash memory or backup memory ICs with a built-in battery, and no socket!) It was very hard not to end up destroying a couple through holes... (and you had to be lucky enough that it would still work.)

You cannot have (cheap) SMD sockets for SMD chips, you have to desolder them.

Well, that's not entirely true. Back in the days where TH components were still popular, PLCC packages started to appear. You could either directly solder them as SMD components, or put them in sockets. So if you're personally still willing to have sockets for your ICs, I would suggest you consider using PLCCs. Of course you won't find everything in that kind of package, but there are still memory ICs, CPLD and old CPUs available. Best of both worlds in that regard.
 

Online wraper

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #53 on: July 29, 2019, 03:17:28 pm »
You just would pay upfront 3x the price for boat anchor which probably would fall apart even before delivered.

You have a problem, and you are looking for a solution.

Before shipping to that company, I asked a lot of guys, colleagues, and friends. Most of them answered that "replacing an SMD chip is a piece of cake", like if it was the canonical sentence to say, but when they saw the motherboards and understood what needed to be done, well at the end of the month, nobody did actually did the job.

I waited a second month, and again I only collected funny answers like "oh, sorry, I am so busy with the new vegetable garden, this year I want to grow onions", "oh, sorry I have to go the dancing club with my wife otherwise she will ask for divorce", etc, and yet again nobody wanted to do the job. I even offered money, and again no one accepted the call.

In the end, two months later, I had two choices: to trash everything and forget it, or to send an email to the company and to be prepared to pay a very salt bill  :-//
If board is so densely populated that it's hard to replace SMD component, there is simply no space for DIP. And it's not a problem of SMD per se. Also as long as there in no bottom pad, you can use soldering iron(s)/tweezers and low melting point alloy to remove the component in complicated areas like nearby plastic parts.
 

Online ebastler

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #54 on: July 29, 2019, 03:25:38 pm »
[...] SMD-flash-chip has a glue/resin on its back, and it's located too near SMD-resistances and capacitors, which makes the desolder procedure a nightmare. 

[...] "supervisor chip" that you usually find in a UNIX workstation [...] usually between two PCI slots, under something, and too near to something that can bend when you apply hot air simply because it's too hard to be protected with tape.

When you can't get in with hot air, there is always the option of destructive removal: Carefully cut all pins at the chip body with a Dremel or such. Then unsolder the remainders of the pins from the PCB one by one with a soldering iron.
 

Offline legacy

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #55 on: July 29, 2019, 04:06:04 pm »
you can use soldering iron(s)/tweezers and low melting point alloy to remove the component in complicated areas like nearby plastic parts.

yeah, that was the main problem, the chip was between the plastic PCI slots, and near the plastic of a SIM72 connector, so I preferred to pass.

I didn't have any appropriate tweezers, and I have never used low melting point alloy. I saw the hot air was going to damage the plastic, so I stopped.
 

Online wraper

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #56 on: July 29, 2019, 04:23:26 pm »
I saw the hot air was going to damage the plastic, so I stopped.
FWIW many plastic parts, especially SMT, survive hot air just fine unless you set too high air temperature. Preheater helps a lot. Also I often cover them with DIY covers from solder paste stencils I no longer need.
 

Offline legacy

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #57 on: July 29, 2019, 04:27:16 pm »
Anyway, why the SMD-flash on the PDA was attached to the PCB with resin? That's really evil!

Why do you need glue or resin? I experimented a true nightmare on a second already dead PDA ...  the typical lead-free solder has a melting point around 220 degrees C, so I did preheat the board close to 200 C and continued with the hot air gun, but when I tried to remove the chip, I found there was "something" under the chip, and it was working as glue. Then I realized that it was a thin film of resin, or something, and it was so resistant to the hot air that I was not able to remove the chip, even if I used a lot of force on the extractor with the hot air gun at 300 C degree.

In the end I destroyed the PCB, to check what was wrong. And I am still shocked to have found a kind of glue under the chip. I still do not understand the reason for this.

I have removed the same SMD-flash chip on my Motorola EZ development board. Thanks to a "relaxed" design (I mean the board is really large, 110x100mm, with a lot of distance between components) there is a lot of space around the SMD-flash, no plastic near the area, and there is neither resin nor glue under the chip, so I didn't experiment so many troubles!
 

Offline SiliconWizard

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #58 on: July 29, 2019, 04:35:40 pm »
I saw the hot air was going to damage the plastic, so I stopped.
FWIW many plastic parts, especially SMT, survive hot air just fine unless you set too high air temperature. Preheater helps a lot. Also I often cover them with DIY covers from solder paste stencils I no longer need.

Oh, SMD plastic packages survive it fine usually. Sometimes the very small packages will tend to degrade if you heat them a bit too much though. But I'm talking about the UDFN or very small BGAs, and there's usually little reason to overheat them as they will desolder easily. (Sometimes when glue has been used to assemble them though, that's a pain.)

Many connectors made of plastic do not like heat as much though. Some PCI/PCIe slots are very sturdy, others will melt.
 

Offline luiHS

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #59 on: July 30, 2019, 02:05:29 am »
yeah, that was the main problem, the chip was between the plastic PCI slots, and near the plastic of a SIM72 connector, so I preferred to pass.

I didn't have any appropriate tweezers, and I have never used low melting point alloy. I saw the hot air was going to damage the plastic, so I stopped.


To protect heat sensitive components, you can cover them with kapton tape.

« Last Edit: July 30, 2019, 10:45:50 am by luiHS »
 

Offline legacy

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #60 on: July 30, 2019, 10:33:25 am »
To protect heat sensitive components, you can cover them with capton tape.

[..]
Now, the problem here is that the chip is usually between two PCI slots, under something, and too near to something that can bend when you apply hot air simply because it's too hard to be protected with tape. Moral of the story ... it usually ends with a cry.
[..]

Kapton tape
« Last Edit: July 30, 2019, 11:46:38 am by legacy »
 

Online wraper

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #61 on: July 30, 2019, 10:38:50 am »
To protect heat sensitive components, you can cover them with capton tape.
Kapton. It's heavily overused on those pictures though. Using it over small components can be even counterproductive. It may stick to to them and when solder melts, lift them from PCB.
 

Online T3sl4co1l

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Re: CPLD to replace 74xx chips in an old circuit
« Reply #62 on: July 30, 2019, 08:08:54 pm »
It also may not do much to components that need it -- plastic connector housings aren't terrifically conductive by themselves, so the Kapton(R) tape is merely comparable in conductivity and doesn't end up doing much.  Layers are needed, perhaps alternating foil and film, to make something more like a "space blanket".  A pocket of air around the connector, paired with a fan to deliver cooling air, would also be helpful.

Note that you really need the high temperature silicone adhesive tape -- the cheaper (and stickier) acrylic adhesive melts and decomposes at soldering temperature.  Any gaps in the barrier let hot air through, melting holes in nearby connectors.

Tim
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Electronic design, from concept to prototype.
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