Do epoxy potting compounds harm my crystal oscillators?

[king.oslo]

I was searching the web to learn if the shrinkage of potting compound could harm my crystal oscillators. A company recommended avoiding potting crystal oscillators, however on the microchip forum, the engineers suggested that potting was not going to harm them. Somebody on the forum also suggested to choose an oscillator package suitable for plotting.

What do you reckon?

http://no.mouser.com/Search/ProductDetail.aspx?R=AA-16.000MALE-Tvirtualkey57230000virtualkey717-AA-16.000MALE-T

I am interested in the above oscillators, do you reckon the potting compound will destroy these?

Thank you for your time.

Kind regards,
Marius

[poptones]

If you don't care about appearance (ie you're not using clear lucite or something), drop a few slivers of wood in the potting around your crystal. This allows for expansion without interefering with the potting.

[jeroen74]

Depends on the potting compound you are going to use. If it's one that cures into something as hard as rock, I would not be surprised the ceramic housing of the oscillator module cracks with temperature extremes. Probably no issue if you use an (PU) compound that stays relatively soft. It too depends on the environment your widget will operate in. If it always operates inside a house, it does not suffer as much as when it's mounted inside a car's engine compartment.

I guess a true hole mounted metal canned oscillator cares way less about all this.

The previously suggested two layer approach should work too.

[SeanB]

Use a potting compound that is flexible, and then you will have no problems. If you need a tough coating then consider using a flexible layer over the board to give a 2mm or so layer and then when this is cured place the hard cure epoxy over it as protection.

[MartinX]

Hard epoxy potting compounds are an evil legacy from the seventies, they work reasonably well with all trough hole components, but with the inflexible surface mount components the different coefficient of expansion of things on the board means it only takes a little thermal cycling and things will start breaking. Larger boards are more sensitive than smaller. Epoxy resin was thought to be the best because of it´s resistance to a wide variety of aggressive chemicals. Today´s softer compounds may not provide the same all-round protection so it is important to choose a compound that withstands the specific environment your product will face.

[king.oslo]

Hello again,

Thank you for your replies

My design is all smd. The temperature fluctuations where the device will be installed in the engine compartment, is high. -40 - 125*C.

1 Will placeing a shield around the oscillator fix the problem, or is the epoxy going to kill the other SMD components as well?

2. Is the potting compound so watery, that it will find its way under the shield, and fill that cavity? In other words, is the viscosity high enough to keep out of small gaps?

3. I have a hole in one end of the enclosure where the connector tabs poke out. This is where I inject the potting compound. This compound has to be pretty hard when cured, otherwise the end user may insert things by accident. Are you able to recommend a compound which is hard enough to prevent the end user to insert items into this end, whilst simultaneously not destroying my smd components? Because it is visible, black is preferred.

4. Where should I buy low volume of potting compound?

Thank you for your time.

Kind regards,
Marius

[MartinX]

With that temperature range and in that environment it is not a trivial task to design a reliably working circuit. Your questions are impossible to answer without more detailed information about your project.

Forget using hard epoxy, vehicle component manufacturers have all moved away from that in favour of softer compounds. I have seen cases where the components were set in the potting compound and not on the board after some aggressive thermal cycling, the components were simply sheared of the board. When the components were all trough hole the leads could always flex a little but with surface mount components any mechanical force on the component means there will be an unhealthy stress on the solder joint.

If you are worried about sharp objects penetrating the potting compound construct some sort of plastic guard plate that allows the connector tabs to poke out and protects the exposed potting compound.

ELFA Distrelec have some potting compounds in small volumes, I have no experience with the ones they sell though.

https://www.elfaelektronikk.no/elfa3~no_no/elfa/init.do?toc=19114&name=tetningsmasse

[jeroen74]

Note that some (or maybe all) two component compounds that come in tins, need to be mixed with a vacuum centrifuge if you don't want the mix be full of tiny air bubbles. Unless the stuff is really thin like water I guess.

The one silicone compound I worked with was really thick (but flowed beautifully).

The datasheet should have processing recommendations.