PC board thickness tolerance

[RobK_NL]

Hi everyone,

My first post here.

I need to get some boards manufactured with a specific thickness. 4.7mm +0/-0.1mm, to be precise. There is a company here in The Netherlands that has made
several hundreds of them in the past, but they are not exactly the cheapest, so I decided to shop around a bit.

The problem is: Nobody seems to be able to do this. I get offers for 4.7mm +/-10% and the best I had was 4.7mm +/-0.25mm 

This seems incredible to me. After decades of board manufacturing, one would think the processes involved are honed to perfection, but evidently this is not so.

Is there anyone with experience in board manufacturing that can shed any light on this?


Thanks,
Rob

[Gyro]

For normal thickness of board stock from a particular manufacturer would be extremely consistent, the trouble is that the board house is probably sourcing from several manufacturers and are covering themselves...

You indicate 4.7mm thickness, which is a lot thicker than standard 1.6mm stock - If you're talking multi-layer then I guess there is more opportunity for finished variation in the multiple pre-preg thickness when layering the board stack. I'm not sure if 4.7mm is a standard stock thickness, they may be trying to laminate boards to meet your specific requirement from the nearest common sizes.

Just a few ideas anyway, maybe you need to ask them few questions to understand their issues in meeting a closer tolerance.

[RobK_NL]

You indicate 4.7mm thickness, which is a lot thicker than standard 1.6mm stock

Yes, this is a decidedly non-standard thing 

If you're talking multi-layer then I guess there is more opportunity for finished variation in the multiple pre-preg thickness when layering the board stack. I'm not sure if 4.7mm is a standard stock thickness, they may be trying to laminate boards to meet your specific requirement from the nearest common sizes.

No, it's not a multi-layer, but they do have to build it from pre-preg. And there's the thing, the pre-preg is available in several different thicknesses; 0.078, 0.12 and 0.18 mm. So to reach 4.7mm, they're going to have to stack some 26 layers or so. With that much room for tweaking, surely it would be possible to get it right. Make a few test pieces to get it spot on, I would gladly pay a bit extra for that.

maybe you need to ask them few questions to understand their issues in meeting a closer tolerance.

As none of them seems willing to let me talk to their production engineers, I thought I'd be better off asking the questions here

[ITman496]

I can't offer much help other then maybe considering getting a normal pcb, making sure it's single sided load (or simple double sided load) and laser cut / mill out a spacer slice of material and glue it onto the back of the pcb, with cutouts for any stuff on the back, to make the thickness correct?

And if you can say, since I'm curious, what aspect of it requires the strange thickness?

[Fungus]

Why do you need that thickness?

Seems like you could save a lot of trouble by going with a standard thickness instead.

[Gyro]

Sorry, meant to say welcome aboard. 

Presumably this is a mechanical loading issue... and existing equipment fit.

I can see the tolerances mounting up with 26 layers of pre-preg!    Yes I'm sure they could fine tune it if with test runs if you make it worth their while but you'd need the volume to justify it. You may need to search out a more accommodating company.

The though occurs to me that 3 x 1.6mm = 4.8mm. Maybe you could do something with laminating board stock (you have two glue line thickness to consider of course). Just a thought. Even two layers of stock with pre-preg between would help with the tolerances - your own non-etched multilayer PCB (Edit: pretty unconventional stack-up though )

[RobK_NL]

I was afraid that would happen

As this is for a client and I do not know whether or not it would count as 'sensitive', I am not going to divulge any information. Rest assured that if there was an easier way
of doing this, we would have done it long ago.

@ITman496
Your suggestion sounds good for a one off, but we're talking about hundreds. And they need to be double sided as well. So you'd have to sandwich the spacer between two single side boards, which you'd
have to position very accurately to get proper registration of the top- and bottom layers. Ehm, no, thanks.

@Gyro
Thanks for the welcome. Glad to be on board

Yes, I agree that with so many layers tolerances would mount up. Then again, it also gives you that much opportunity for tweaking. At least, that's my slightly naive take on it, I am hoping someone with more knowledge will chime in and set me right.

[Gyro]

Hmmm, not sure I could claim to be more knowledgeable on board layups (other than to say that 26 layers of pre-preg isn't the way they're supposed to be used), a few layers at most to control impedances in multilayer boards. You might be best looking for a house that does mil-spec boards and contracts, they will be more experienced (and hopefully more willing to give advice) in 'odd-ball' requirements like this. It might cost you though.

P.S. I'd try to arrange to visit them if possible, they're more likely to be helpful face to face, show you other work etc.

[Fungus]

Presumably this is a mechanical loading issue... and existing equipment fit.

Mechanical loading might need a thick board but it wouldn't mean such tight tolerance.

'Fit'? You mean sliding into a tight slot or something? Just specify a tiny bit less, eg. 4.6.

[RobK_NL]

It might cost you though.

I can have them made and it does cost me, hence my search for an other board house.

The company I mentioned in my starting post has made ~1000 of these things over the past decade. But they only do it by cheating, i.e. making some 250 or 300 and measuring each and
every one to be able to deliver the required 200. That puts the price right up, I can tell you.
The last order for 200 was a year and a half ago and their most recent quote is 15% higher.

'Fit'? You mean sliding into a tight slot or something? Just specify a tiny bit less, eg. 4.6.

That would still leave the tolerance. 4.7 is the absolute maximum, we could live with 4.5 minimum. But as I said, the tightest tolerance I got is +/-0.25mm, so to get 4.7 max, I would need to specify 4.45 and the minimum would go to 4.2 

[Gyro]

But they only do it by cheating, i.e. making some 250 or 300 and measuring each and
every one to be able to deliver the required 200.

Ouch! That's painfully bad failure rate. Even with a lot of practice it sounds as if they can't keep their processing pressure and temperature constant enough - either they are really bad at it, or it really is that hard.    Maybe you should just get some sample runs done by other suppliers (without stressing the tolerance too much) and see how they measure. The only other way to improve on it is to find a way of including more thick cores to minimize the number of interfaces, maybe a competitor can do that.

Good luck, at least you have plenty of motivation with the cost increase! 

[daqq]

It's a pretty non standard operation. Try these guys:

http://www.sqpinternational.com/

They did some precision thingie for me in the past. Also, these guys do special:

http://www.cube.cz/?lang=en

Could you adjust your process? You could have a 5mm +/- 0.3mm board made with the components placed on an exposed inner layer ( like this http://www.sqpinternational.com/images/sqpinternational4.jpg ) then mill off the rest of the PCB on an actual mill.

[Ice-Tea]

Have you checked with V-PS yet? They have always been open in their dealings with me, I'm sure you can have a decent technical discussion with them.

[KL27x]

Maybe it would be cheaper if you ordered the boards and sorted them yourself with a pair of calipers. Then send the good ones back out to an assembly house and toss the rest.

At +-0.25mm, depending on the distribution, that might be similar to what your current fab is getting. With an even "white noise" spread, that would be only 40% yield. But with a bell curve, that might give close to the 67-80% you have described from your current fab.

It would be a rather expensive thing to find out, if it's a total bust. (and all the boards come back either +.25mm or -.25mm, lol)

[RobK_NL]

Thanks for the additional suppliers guys (and/or girls), I appreciate it. Let's see what they can offer me.

It would be a rather expensive thing to find out, if it's a total bust. (and all the boards come back either +.25mm or -.25mm, lol)

Yep, several thousand euros, so I'll not be going that way any time soon 

[Someone]

Top end board houses can offer high precision on board thickness (RF impedances and edge connectors rely on it) by pressing the panels to the required thickness as the epoxy sets, not sure what pricing you are targeting however.

[ConKbot]

Guessing its not just for slots on the edge where you can have the edges milled to the right thickness?  Or have critical, mechanical only spots machined to the right thickness in a post fabrication, pre-assembly step?

I definitely wouldnt bet the thickness to be a gaussian spread, all layups/panels they do in a single pressing will probably come out pretty close to each other. 

[Fungus]

'Fit'? You mean sliding into a tight slot or something? Just specify a tiny bit less, eg. 4.6.

That would still leave the tolerance. 4.7 is the absolute maximum, we could live with 4.5 minimum. But as I said, the tightest tolerance I got is +/-0.25mm, so to get 4.7 max, I would need to specify 4.45 and the minimum would go to 4.2 

Can't you order 5mm boards then get somebody to mill just the edges down to fit the slot?

[Monkeh]

Does it actually need to be that thick all over, or is it just a case of fitting in a slot?

[RobK_NL]

The "slot" is actually the distance between the pins of a component (72 of them, to be precise) and these need to be soldered to the top and bottom layers. So any
post-production thickness milling is out of the question.

[Ian.M]

Talk to a board house that offers plated through slits and see if they can do depth controlled plated V grooves by the same process.

[daqq]

Perhaps changing the process to first creating the laminate (to, say, 5mm +/-0.3mm), THEN milling it on a normal mill to exact values, then plating, etching etc? it would be useless for more than two layer boards though. Probably.

[Fungus]

The "slot" is actually the distance between the pins of a component (72 of them, to be precise) and these need to be soldered to the top and bottom layers. So any
post-production thickness milling is out of the question.

Aha! Finally we get some useful information...

(although I wouldn't have thought that soldering needed such tight tolerances  )

[ConKbot]

The "slot" is actually the distance between the pins of a component (72 of them, to be precise) and these need to be soldered to the top and bottom layers. So any
post-production thickness milling is out of the question.

Aha! Finally we get some useful information...

(although I wouldn't have thought that soldering needed such tight tolerances  )

I wouldnt either.

Is there another mechanism to support the connector? There should be, and you should fine as long as there is a healthy solder fillet on the pins to the pads.  Spec it so that the high end of the +0.25mm tolerance would be touching the pins on both sides, and then if it comes out at -0.25mm, then its still only 0.25mm/10 mil per side, which is perfectly acceptable to solder imo.