Board houses that do assembly and support 3 mile track and clearance rules

[perdrix]

I want to get a 4 layer board made up (all parts fitted) - including PIC32MK0512MCJ064-E/PT and TLS850C2TEV33 - don't want to risk "dodgy" parts.

The board has a 3 mil guard trace between two pads of a 0.5mm pitch TQFP64 footprint, so 3,3 rules are needed.

I'd like your recommendations for good board houses to do this for me as I don't fancy hand soldering the 0.5mm pitch TQFP64 PIC.

I am based in UK, so good UK board houses preferred.

David

[Doctorandus_P]

A clearance of 3mile is quite big, I have never seen a PCB of that size. 

I also have some question marks around using guard traces around microcontroller pins, although this one has built in analog opamps.
But even so, how good are these opamps and how much noise is coupled in from the uC itself?
Guard traces are usually useful in high impedance circuits, while this IC is geared towards motor control, which is a noisy environment, but also low impedance.

I had a look at the design rules from Aisler (Because they also do assembly):
https://aisler.community/t/pcb-design-rules/41

But for 4-layer PCBs they only go to 125um and you want about half of that.

TQFP's are still quite easy to solder manually. It's usually a combination of accurate positioning (Use a stereo microscope) and then use lots of flux and drag a big blob of solder over the pins.
If you go manual, then you could do some extra things. For example, maybe you can remove some pads to make room for your guard traces, and then cover them with both solder mask and silkscreen. Nobody in their right mind would give a guarantee for this, but with two layers of paint in between it probably works. You can even lift or break off some pins. Maybe just configuring them as high impedance inputs is already good enough and you can then make those pins part of your guard net. (the guard tracks are (should be) driven by a fairly low output impedance).

[exmadscientist]

If you shave down the pads, this isn't too hard to do with 4/4 rules. Yes, you'll end up with a couple of pads that are "too small", but it will yield fine on a 0.5mm pitch QFP if it's only 2 out of 64 and you're not making millions of units.

With a full-service fab it is also decently common to tell them "hey we have this one spot with tight rules but most of the board is pretty coarse" and they can usually push their process for you without much impact. (I believe this gets much harder if there is more than one area of special concern.) This usually happens with BGA breakout but this would certainly qualify as well.

[srb1954]

I also have some question marks around using guard traces around microcontroller pins, although this one has built in analog opamps.
But even so, how good are these opamps and how much noise is coupled in from the uC itself?
Guard traces are usually useful in high impedance circuits, while this IC is geared towards motor control, which is a noisy environment, but also low impedance.

The internal op amps aren't anything special in term of performance as evidenced by their mediocre CMRR and PSR. The input bias/leakage currents are also quite high which is probably due to the extensive multiplexing of pins onto the chip. The performance of the op amps is therefore limited mainly by internal coupling within the chip and the addition of external guard traces won't be of much benefit.

[JPortici]

I also have some question marks around using guard traces around microcontroller pins, although this one has built in analog opamps.

It's (probably) for the oscillators. It's really, really tricky to make them work with crystals in PIC32MZ/MK. Why just not use MEMS oscillators, i say

[perdrix]

It was guard tracks for a crystal and I have changed to an external oscillator.

Thanks
David