100A pkg question.

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Hello,

I mfg. a product that use an ATmega128 in the 100A pkg. With a little practice we are able to solder the part onto our boards by hand. The problem is we have had several units that have lost connectivity over time. I know these part should be potted but I have a couple of questions.

- Should the whole part be potted including pins?
- What materials are recommended for potting?

A

AVR Studio 4 Ver. 4.18 684
avr-gcc Ver. 4.3.0
ISIS 7
ELECTRA

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I can't see the 100A package in the data sheet. What is it?

Leon

Leon Heller G1HSM

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Did you mean the Atmega1280? Why do you think it should be potted?

Keep in mind that if you have soldering deficiencies on your circuit board you can still loose connectivity after potting, but the hardened potting compound will prevent any chance of a repair if it covers the solder joints.

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Don't see any 100A package on Mega128 but do on 1280.

It is a gull-wing TQFP. You should be able to inspect with a magnifier and do an adequate job of soldering so that its not an issue. Lots of flux.

I have never seen a recommendation to pot. You really need to get good soldering first, before anything else, because solder joints can fail, even if you do cover it up. Then, its a ^*&Y*Y*( to fix!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Guys,
Thanks for the feedback. You are correct of course, I am refering to the ATmega1280. We have only been using rosin core solder. The part seems to be attached but we do occasionlly have a pin brake lose. We are using standard weller irons should we be using something else?

A

AVR Studio 4 Ver. 4.18 684
avr-gcc Ver. 4.3.0
ISIS 7
ELECTRA

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Your soldering technique is probably to blame. I use drag-soldering with a Metcal mini-hoof tip and don't have any problems with fine-pitch devices, even on home-made PCBs.

Leon

Leon Heller G1HSM

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Leon,

I will have to have look at this. Do you have a link to NA distributor.

A

AVR Studio 4 Ver. 4.18 684
avr-gcc Ver. 4.3.0
ISIS 7
ELECTRA

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You can do drag-soldering with any iron, although it isn't as easy. Metcal is here:

http://www.okinternational.com/

I use a second-hand STSS power unit with a new MX-500 handpiece and cartridges. It cost about the same as a new Weller soldering station. The new MX-5000 looks very nice.

I don't think that Weller makes 'hoof' tips. Pace and one or two other manufacturers also make them.

Leon

Leon Heller G1HSM

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Thanks Leon

A

AVR Studio 4 Ver. 4.18 684
avr-gcc Ver. 4.3.0
ISIS 7
ELECTRA

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I think you need some good flux also. Either the liquid in a flux pen, or the paste in a syringe. It prevents solder bridges, among other things.

You say a standard Weller iron. Weller makes all kinds of irons. You should use a temperature controlled iron.

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I'm assuming you know how to solder. The soldered pins must look bright, shiny and even. If the soldered surface is dull or irregular you aren't doing it right. Try using a different brand of solder wire, or get a fresh batch if your current batch has been lying around too long.

How fresh are your PCBs? Are the areas that will be soldered bare copper? Sometimes if you leave them lying around too long they get a little difficult to solder.

Use tinned PCBs - only costs a little extra; some PCB houses do it for free.
or
Pre tin the PCB before soldering. This means solder the PCB pads first, then put the IC there and then solder the pins.

Also, avoid touching the IC pins and the PCB with bare hands. Even clean hands have a film of oil and sweat.

Is your PCB subject to flexing after it has been assembled? See what you can do to prevent this.

If you think education is expensive, try ignorance.

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steve17, emuler,

We have been stuffing boards here for a while. (20yrs). We have always avoided SMT hardware for this very reason. Our boards are new and tinned. We are using temp. controlled irons. I have not tried any external flux because we have been using rosin core solder.

Our boards are secure but they are subjected to some temerature changes. ( -10C -> +30C ). This does not happen often but it does occur.

I really want avoid paying an outside house to put this stuff together so I will try some of the things you have suggested.

Thanks for the input.

A

AVR Studio 4 Ver. 4.18 684
avr-gcc Ver. 4.3.0
ISIS 7
ELECTRA

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You should also use solder which contain 2-3% silver. Kester 285, for example.
Big difference from normal 60/40 solder.

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Got it,

Thanks Thomas.

A

AVR Studio 4 Ver. 4.18 684
avr-gcc Ver. 4.3.0
ISIS 7
ELECTRA

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andrew99 wrote:
. I have not tried any external flux because we have been using rosin core solder.
I use rosin core solder but I coat the pads with flux when soldering SMT. I don't think I get enough flux just from the solder.

If you suspect a bad solder joint, you can coat the leads with flux and drag the iron across the leads. You should end up with a nice uniform shiny coat of solder on the leads. You definitely need flux for the drag operation. The flux increases the surface tension of the solder. The molten solder tries to form a rounded smooth surface. It won't bridge the pins.