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Requesting advice on SMT vs. Through Hole

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rberek

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Hi

I am helping to add functionality to a product produced by a friend's company. The added functionality is some pretty simple and straightforward low-speed logic and would implemented as an additional PCB in the product itself.

This product has to endure a rough duty environment in extremes of temperature and must have a low EMI footprint. The product will be standards-tested to ensure it meets the requirements (MIL-spec).

Given that, I elected to produce the prototype with discrete logic rather than a microcontroller, to reduce EMI , and with all through-hole components for added mechanical strength. And it works as desired in testing.

Now, I have to think about productizing it. It is, to no surprise, bigger than I would like, so I was thinking about utilizing SMT devices as much as possible to shrink it. But I am worried about the board becoming more fragile. I should note that there is already an off-the-shelf electronic assembly in the current product that has SMT construction, so the overall man product will still have a surface mount assembly in it regardless of my choice. However, this commercial component has not yet been subjected to the rigorous testing required.

Ultimately I will produce an surface mount version of the board and test it long with the current one, but I thought I would ask in advance for opinions regarding the usage of SMT on rough duty environments. Perhaps the answer is "test it and see" but I thought I would poll the experts first.

My day job is a chip designer for equipment that sits in environmentally controlled rooms so this subject does come up much around the water cooler...

Thanks in advance.

r.b.
 

barry

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SMT parts will generate less EMI, simply because shorter trace lengths will be involved.

When you say "rough duty" is it simply temperature extremes (what are they?), or are you also concerned about shock and vibration?
 
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ads-ee

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Given that the requirements are MIL spec then all of the above, temperature -55 to 125 C, shock (drop test), vibration (sit at the resonance frequency if its within the test frequency range) are probably applicable.

It's been a long time since I worked on boards for the mil sector, but I remember we did make a MIL spec board that was entirely surface mount, and it survived all the tests, it was made with something other than FR4 but I don't recall the material. The material's expansion coefficient was nearly identical to the ceramic packaging of the LCC components. Also the board was much more rigid than an FR4 board, of course the 18 layers would have contributed to the overall stiffness. It also was still using lead solder so no cracking of the solder joints under vibration and thermal shock.
 
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marce

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Do class 3 boards for to MIL spec and above, SMD components FR4 boards no problem... Just pick components with the correct temperature parameters and a decent high Tg high Tdl laminate. If it is truly MIL then lead free solder will have to be used.
Very few ceramic devices used these days, generally for near space and space applications, again here quite often the base laminate is FR4.
Components not to use are SMD electrolytic caps, you tend to find them in the bottom of the case after vibration and ballistic tests.....
 
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rberek

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Hi all

Thanks for the replies!

Yes, the product will have to go thorough all of the tests that ads-ee listed. I apologize for for not being clearer.

I appreciate the info on EMI and board and component choices. I knew that mil-spec parts used SMT but a lot of the info I read raised concerns about the shock-resistance of the smaller SMT parts because of the tiny adhesion surface of the pads to the PCB. But clearly you folks have made it work work with smart materials coices, and that is good to know. Thanks marce and ads-ee.

As far as EMI goes, I had shied away from a microcontroller partly because I did not want an oscillator running on the board, and partly because it would be overkill for the application. But SMT would help lower the EMI as barry kindly pointed out.

At any rate, this gives me impetus to get an SMT prototype out earlier and get it tested. Thanks again.

r.b.
 

marce

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Actually the smaller SMD parts (apart from the caps I mentioned) are better than PTH devices for withstanding vibration, even though the pads can seem small the adhesion areas to component weight is often high, plus you get less differential vibration.
Have fun....
 
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rberek

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Thanks again for the help.

I was wondering about that myself when I was reading it. The mass of the parts is so small, and their center of gravity is so close to the board that it would seem they would not be affected as much by vibration and shock.

r.b.
 
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