shield can

The Laird ones are available in 10mm height.

Cheers

Phil Hobbs

Den mandag den 12. december 2016 kl. 22.28.59 UTC+1 skrev John Larkin:

We've had custom cans made, they were etched and then bend into cans the custom tooling for the bending is the expensive part so you need to order a few 100 to not make it silly expensive

looks like these have standard sizes up 15mm deep:

The dip OCXO is big. If I put it in an IC socket, the box needs to be about 0.75" high. I could put in those little spring socket things and get the can height down to 0.5" maybe. I need some air gap above the OCXO to minimize heat loss.

Air flow around the OCXO almost doubles its power drain. I bet it trashes phase noise too.

Just for thermal?

How about some foam, like EPS, EPE or EPP? Glue it on, or make some mechanical feature to hold it on (two or four nylon bolts would do it, slip them into holes in the foam).

Make a block with a cavity and affix it over the OXCO. Won't give you any electrical shielding at all, but it'll give you good thermal shielding.

To make the block with a cavity just make a rectilinear donut out of the correct thickness of material, then glue on a foam "lid". If you've got someone local that does custom foam packaging they'd probably be able to do it for you.

or something like this:

throw away the flat part and mount with screw through pcb

Pretty much. I probably don't need electrical shielding.

We did some experiments. In a small volume, a box with just air inside is a better thermal insulator than a same-size box of foam, or a box with added foam inside. Foam conducts heat better than still air!

Leader Tech makes the cool surface-mount pick-and-place box clips, 14 cents each. They just quoted me a custom shield box, Alloy 770, that will pop into the clips, under $5 at 100.

I wonder if I could push a plastic potting shell onto those clips.

Packaging is sure a nuisance.

Why not use polyurethane structural foam? You squirt it out of a can into a - say - folded paper mould, wait a few minutes until sets, and strip off the paper.

It really will keep the airflow off. It doesn't look good, but you could probably paint it to make it look better.

How about some foam... Melamine goes above ~150C.

George H.

Sure but it doesn't take that much temp difference to get convection started. George H.

Why remove the paper? I will say the polyurethane foam is a PITA to work with. Hugely sticky and hard to remove. The stuff I got foamed up for several minutes after I sprayed it into a space. It ended up dropping out of the opening making a big mess. I'm not sure how you would fill a small box without the same thing happening.

Better would be a sculpted plastic foam box. I'm not at all sure why JL wants to try to optimize the heat transfer so much. The difference between air and plastic foams is so small it's hardly worth worrying about. I expect the PCB will conduct far more heat than the insulated portion of the box. He said he made measurements, but I can't imagine the insulation is the limiting factor.

It's hard to argue with that.

Or find something that'll work. Someone (Lasse?) posted a little plastic project box with two screw holes that'd work. If 3D printing could be had for $5/ea in the hundreds, you could have a box made -- if you could find someone to design it.

How often do you do it? I've known some mechanical engineers that just love it, are good at it, and treat a project with difficult packaging issues as a blessing rather than a bane. If you're burning a lot of EE hours doing packaging, maybe you need to find someone local that likes it.

Constantly. A PC board in a box needs dimensions, mounting, cooling, connectors, shock and vibration tolerance, test access, all that. PCB layout *is* packaging.

I've known some mechanical engineers that just

MEs who do good electronic packaging are rare.

Why do MEs love to hide fasteners? Are they ashamed to admit that they use screws?

And why do they like symmetric hole patterns that encourage people to assemble things wrong?

Making things out of stuff is labor intensive. Molding, casting, cutting, gluing, painting, machining are messy and expensive. There are millions of parts out there, cheap and done, so the best design selects some of them.

As noted, in a small volume air is a better thermal insulator than foam. Cheaper, too.

They've probably been trained by marketing guys who think screws are ugly.

So, challenge the guy with "make it so it can't be put together wrong"

And assume that concept has never occurred to him?

On Tuesday, December 13, 2016 at 10:38:29 AM UTC-8, John Larkin wrote: [about putting a box around an ovenized XO]

No biggie. You can vacuform a sheet of plastic that will do this job nicely, and the ancillary operations on a 3x4' sheet of boxes just takes a few cuts with a shear and (worst case) punching two holes or (better) applying some double-stick foam tape. Any sign shop can get the job done before lunch.

It used to be, every electronics supplier had shielded octal-mount enclosures that would be perfect to hold an OCXO and insulation and shielding. Nowadays, there's easier options. Seymour Cray famously ticked of the twelve guys you have to have, to build a computer. Number one was the mechanical engineer, for the box; thing is, you need him/her for the air shields, too.

Haven't we all seen failures that happened because the box was just... wrong? Heard about the mechanical clearance issue that sank the Galaxy Note 7?

Yup, of course I've also spent lots of time looking for the right part. Half the battle is to know what something is called.

George H.

Some 3D printed parts are *astoundingly* cheap - even 1 off. I've had something like a "thick thimble" made in SLA (

le

he

it

The small volume has to be small enough that the Raleigh number is less tha n about 600

For double-glazed windows, increasing the gap between the panes above 9mm d oesn't provide extra insulation because convection starts setting in. For t hree dimensional hollow objects the maximum dimension is a bit higher, but the Raleigh number for a sphere increases as the cube of it radius.

Closed cell foam isn't much worse as an insulator than still air, and it is a lot easier to manage. What you really need is a closed cell foam with ro ughly 1 cm diameter thin-walled cells.