how to validate that a unit is sealed 4

[dgowans]

Our products generally consist of a PCB assembly that gets sealed in a plastic enclosure (roughly 8"x4"x1.5") via a hot-plate welding operation. Our current method of verifying that the welding operation produced a fully sealed unit is to heat the sealed product for a period of time sufficient to build internal pressure that causes the product to expand - the 1.5" dimension increases. This is a time consuming process that we're trying to revise or eliminate. Additionally, our new products are continually getting smaller so even if we wanted to stick with the same test method the amount of "puff" seen in the new enclosures is pretty minimal, making evaluation difficult.

We have proven that we can perform the same test by evaluating the units in a vacuum bell, but the equipment is quite expensive and there is resistance to going this route.

Does anyone have any other suggestions as to how we can perform this testing? Obviously fast and inexpensive are preferable, but at least we don't need a solution immediately.

Thanks in advance.

 

[potteryshard]

Put a gas port in the plastic enclosure, and pressurize the entire assembly with nitrogen? Pressure can be checked at a later time/date.

 

[dgowans]

potteryshard,

We've toyed with that idea, or something similar, but didn't go too far because of cost. In any case, thanks for the suggestion.

Additional useful information for anyone else thinking about this is that the devices will be made by the millions and of course are cost-sensitive.

 

[PeterStock]

Use a distructive test on a number of units per lot. Drill a hole in the case, then pressurize. If a test piece fails, scrap the lot.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[HDS]

It sounds like you are looking for an air tight seal. A pressure chamber instead of a vacuum chamber would give the same results but be easier to operate.

 

[MiketheEngineer]

If you feel that your product is usually done right - say 99.9% of the time - you might just ignore the whole process and live the returns??

But I would still do a spot check from time to time....

 

[IRstuff]

If it's truly pressure sealed, then a straight helium leak test should work. It requires that you pressurize the enclosure in high pressure helium. If there is a leak, the helium will enter the case, and then, in the helium testing phase, you detect whether the helium that got into the case is getting back out. Obviously, you'll need to establish test criteria, etc...

TTFN

FAQ731-376

 

[MikeHalloran]

A windowed pressure chamber with a dial indicator zeroed on the center of the largest surface should suffice.

Dry air should be fine.
Nitrogen is an asphyxiation hazard and requires special precautions.


Mike Halloran
Pembroke Pines, FL, USA

 

[Twoballcane]

Can you back fill the bag with nitrogen gas (this will displace the air to keep the unit dry) and then give it a quick dip in water to see if there are any leaks? Or back fill with helium and use a helium sniffer to sniff out leaks?

Tobalcane
"If you avoid failure, you also avoid success."

 

[btrueblood]

A bell jar would certainly be an expensive and slow test process.

But I'm sure a competent mechanical engineer with access to machine shops could build and/or buy equipment to make a vacuum chamber "tool", complete with quick-clamp lid, integral displacement sensor and a high-speed vacuum pump, for not too much money.

Even cheaper if you just use shop air and go the overpressure test route, though you then need to be more careful about the chamber design due to explosion/rupture risks.

 

[kingnero]

Can you not simply submerse the item in water, and see if bubbles are coming out? Perhaps with putting external pressure on the plastic box, to force the air out?

 

[prdave00]

What about a leak tester like those from Uson (

http://www.uson.com/)?

These systems pressurize the vessel and then monitor any drops indicating leaks. You still need a port to to pressurize the inside of the unit and, yes, these units aren't always cheap going back to your response to potteryshard (but can be quick for in-process inspection). At my old company we got used ones pretty cheap from eBay and auctions.

 

[Compositepro]

A standard test method that is very fast, cheap, and reliable is to momentarily submerge the enclosure in a bath of warm water. The heat will almost instantly pressurize the air in the enclosure and, if there is any leak, you will see a stream of bubbles that exactly locates the leak. There is very little possibility of getting water inside if you wipe-off excess water after the dip. The warm enclosure will then dry quickly. So simple that 100% inspection is possible.

With this method you get reliable results in seconds. If you are measuring case expansion it is very difficult to detect small leaks. Pressure loss takes time and temperature changes during this time will conceal a leak.

 

[IRstuff]

Again, bear in mind that you need to establish what your acceptance criteria is. Bubbling only occurs if the leak is relatively large by hermetic sealing standards. Only you can determine what the acceptable leak rate is, and act and test accordingly.


TTFN

FAQ731-376

 

[dgowans]

Thanks to all for the various suggestions - keep them coming! We obviously have more research to do but have more avenues now than we did a couple hours ago.

IRstuff, agreed on the acceptance criteria.

 

[HDS]

Weigh them
dip 100 of them in a pressurized water tank
Reweigh them
any that gained weight leaked

pressure will need to be low enough to not create leaks but high enough to go inside.

 

[IRstuff]

In a similar vein, pressure bombing a leaky container would ostensibly change the resonance characteristics of the box.

Have you considered adding a pressure sensor internally? Then, you could easily pressure bomb the case and see what the readings look like afterwards.

TTFN

FAQ731-376

 

[btrueblood]

"Weigh them - dip 100 of them in a pressurized water tank -Reweigh them - any that gained weight leaked"

...or the plastic case absorbed water.

 

[HDS]

They could also be a potential for the pressure to close the leak ...

Things are never as simple as they look.

dgowans please let us know if anything works.

 

[IRstuff]

"Weigh them"

We had to do that, once, on a munition with a leaky coolant vessel; was looking for a fraction of an ounce change in a 30 lb unit.

TTFN

FAQ731-376