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Error rates on pcbs.
Error rates on pcbs.
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Old 29th December 2011, 11:18 PM   #1
nigelwright7557 is offline nigelwright7557  United Kingdom
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Default Error rates on pcbs.

I have bought quite a few pcbs mostly from China and had very few problems.
One problem was resistive tracks on a mixer causing hum. I had to add wires to short out the resistive tracks.

The only actual pcb track fault was a broken track on an amplifier pcb.

I tend to buy in batches of 10 to 25 so I suspect not much testing other than a brief look over is done.
Clearly I need to specify copper ounces to help with the resistive problem and also include a ground plane.

I just wondered if any one else had had any similar problems ?
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Old 30th December 2011, 12:01 AM   #2
Cristi is offline Cristi
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Error rates on pcbs.
Quote:
Originally Posted by nigelwright7557 View Post
I just wondered if any one else had had any similar problems ?
Oh, yeah, i had such pleasant experiences several times already. and the nasty part is that some pcb's look perfect and if you measure them, there are no shorts or interruptions. but after they are assembled the board fails to work. this is mostly due to interrupted vias, or microscopic shorts which are not detected in the testing phase.
when i order a batch of pcb's i require 100% electrical test, not just visual inspection like many pcb companies consider QC. there are few ways to test a pcb. leaving apart the traditional method, where a myopic lady stair over a pcb without founding an interruption even if the board is broken in two , for large quantity the factory use the "bed of nails" this is expensive but very accurate. several voltages can be used to test the board, AC and DC. the most common method to test the boards for small and medium batches (up to 10k pcs) is the flying probe tester, which has two or more arms with test points moving over the pcb and touching the pads and check for continuity and shorts, according to netlist. this is less accurate since it cannot detect the problems which might ocure after the board is assembled.
and of course, the final test, after the board is fully assembled, for medium-large quantities the bed of nails is used, again...
for digital circuits which contains one or more CPU's or PLD's, the JTAG boundary scan test is the preferred method. this allows several devices to be cascaded and tested. also, JTAG interface which became a standard, is used for programming and firmware update in the past decade, without the need to remove the device from the board or even to open the box.
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Old 30th December 2011, 12:12 AM   #3
nigelwright7557 is offline nigelwright7557  United Kingdom
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It is tempting to buy cheap but a fully stuffed board that needs to be thrown out is useless and expensive. The possible permutations of pcb errors is huge.

The first electronics company I worked for bought in cheap pcbs and the etching was poor with loads of shorts. Of course the shorts had to be under an IC which needed desoldering to cut the short. Sometimes desoldering the IC wrecked the pcb anyway.
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Old 30th December 2011, 07:17 AM   #4
Baldin is offline Baldin  Denmark
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Hi nigelwright7557

I have good experience with Olimex. I only buy a few PCBs at a time, and really like the option ot get different pcb made on the same panelized board as long as it fits dimentions.
I have experienced not faults on their behalf yet.

What companies in China have you used and have good experience with? Like to broaden my look

Happy new year Baldin
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Old 30th December 2011, 08:22 AM   #5
dhaen is offline dhaen  Europe
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Default Chinese supplier pcbcart.com

I buy small pcb's by the 100's from pcbcart.com and have yet to see a fault. The only difficulty I've had is understanding exactly what they are asking me - when I've made a mistake or i've asked for something out of the ordinary. I continue to use them and would recommend them.

A number of years ago I used to use a small local (UK) company for big PCB's - 30cm+ and had repeated problems, usually intermittent conductivity of via's or of plate-through pads, occasionally over-etched (thin) tracks. That company went out of business.
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Old 31st December 2011, 04:29 PM   #6
gmarsh is offline gmarsh  Canada
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In my experience..

(1) The general attitude of a PCB place seems to matter more than anything. Some places are really professional and try their best to put out a good product, and others say "hmm, that's strange, we'll look into that" when they ship you a bunch of defective boards. If a PCB house pisses you off, go elsewhere.

(2) Don't push the process. If a PCB house advertizes 8/8 or 4/4 mil trace/space, don't send them a board with a big parallel bus laid out as 8/8 or 4/4 respectively - you can pretty much guarantee that you'll receive a board with those bus lines open or shorted together. Likewise, don't use the board house's minimal drill size, or you'll find yourself with high resistance or open vias.

If you don't have a choice (eg, putting a 0.5mm SMT device on an 8/8 process board) talk to the board house before you place the order. They may suggest changes to your device footprint that may eliminate any potential problems, and if they're a good shop, they'll inspect the first few boards they build and fine tune their process to make sure your board gets made right.

(3) Don't underestimate the cost of testing/reworking/throwing out assembled boards.

If you're building a cheap PCB which is cheap to test, and you don't mind ordering extras and throwing out the odd board, by all means use the cheapest PCB house out there.

And if your board has to be reliable in the field, eg. if it's going in an automotive, broadcast or industrial application, under no circumstances should you ever cheap out on the PCB. Don't push the process, use a better process than you need, and if your components are expensive then get a full electrical test done on the PCBs before assembly.

I literally fought with my last employer about this - they went from a $50/piece to a $30/piece PCB vendor for a multilayer PCB with about $500 of parts on it. Yield dropped substantially, and we found ourselves short on boards and wasting money by discarding boards. I spent late hours fixing boards to meet shipments, we had a couple boards fail in the field which pissed off customers, and as the board designer/"owner" I ended up getting blamed for the whole mess. Thankfully they saw the light and changed back to the original vendor.
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Old 31st December 2011, 08:01 PM   #7
ChocoHolic is offline ChocoHolic  Germany
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Quote:
Originally Posted by gmarsh View Post

I literally fought with my last employer about this - they went from a $50/piece to a $30/piece PCB vendor for a multilayer PCB with about $500 of parts on it. Yield dropped substantially, and we found ourselves short on boards and wasting money by discarding boards. I spent late hours fixing boards to meet shipments, we had a couple boards fail in the field which pissed off customers, and as the board designer/"owner" I ended up getting blamed for the whole mess. Thankfully they saw the light and changed back to the original .vendor.
But for sure this low cost vendor looked great on the slides, which were presented to your management and somebody got a good bonus for that great cost cut.



...so far I only ordered once from a Chinese vendor, basically the board showed good quality.
Good quality, but wrong copper thickness, I had ordered a 70µm base material thickness, but they made in 35µm. And the best was that their reaction when I complained:
Choco: 'My measurements show approx 50µm copper, means 35µm base material +15µm galvanic. I ordered, 70µm base material....'
PCB-Company: ' Trust us. You cannot measure the copper thickness without special optical equipment.'
I did not answer and did not explain 4-wire-measurement...

Another time I ordered from Würth:
WEdirekt ? der Online-Shop von Würth Elektronik | Leiterplatten und Stencils ab 1 Stück | Platine, Leiterplatten, Prototypen, PCB, SMD-Schalter, Stencil, Lötpastenschablone
Perfect, but not cheap.
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Old 31st December 2011, 08:30 PM   #8
gmarsh is offline gmarsh  Canada
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Quote:
Originally Posted by ChocoHolic View Post
But for sure this low cost vendor looked great on the slides, which were presented to your management and somebody got a good bonus for that great cost cut.
Wasn't any of that. The purchasing department found a new vendor which quoted lower prices with the same specifications as the old vendor's process. They felt that they were doing the right thing, and changed vendors without asking engineering.

There wasn't an official "ask engineering first" policy in place at the time, so nobody was ever really was at fault. Didn't take long after the PCB snafu before an approved vendors list system went in.
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Old 31st December 2011, 10:59 PM   #9
ChocoHolic is offline ChocoHolic  Germany
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Quote:
Originally Posted by gmarsh View Post
There wasn't an official "ask engineering first" policy in place at the time, so nobody was ever really was at fault. Didn't take long after the PCB snafu before an approved vendors list system went in.
Handling vendors and knowing when to recheck with engineering, that's in fact more difficult than it looks like!
I am aware of that, but just could not resist to put that sarcastic comment above.
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Old 31st December 2011, 11:04 PM   #10
nigelwright7557 is offline nigelwright7557  United Kingdom
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I was a little naive with pcb quotes to start with, I just asked for the cheapest option.
I got very thin copper on the tracks and the copper with resist scratched off looked a bit black so was probably very cheap copper.
I also should have asked for a ground plane.
Well we live and learn.
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