It's on my to-do list to just post the pdfs on my web page. Thanks for the reminder. Maybe I should get to that this weekend -- Joerg (from SED) mentioned that I should put my recent analog experience up in my "capabilities" section, as well.
Shortly after posting my plea, and entirely independent of it, a whole bunch of short-term work fell into my in-basket. It's that whole Murphy thing -- I'm sure that if I'd kept my fingers off the keyboard that I'd still be looking an one-and-only-one customer right now.
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Tim Wescott
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I could see, if I were hurting for work, writing a clause into the contract that states that inasmuch as the project is technically risky, as long as I follow their specifications they assume all risk.
Then I'd ask for money up front...
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Tim Wescott
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I will say the cross posting has been a bit of a PITA. I read all three groups and if I don't want to miss a reply to just one group I have to read all three threads in all three groups. So far I haven't bothered with this thread in comp.dsp. I wonder if I missed anything there?
My brother is not an engineer and has an interesting project where he is taking a government designed system and commercializing it. He has a mechanical engineer but the electrical engineer isn't working out because he has a full time job plus is moonlighting other projects too. I was talking to him about the project and at one point he said that the board which needs to be redesigned due to parts obsolescence has to amplify the signal, "so the software doesn't have any problems with it", meaning to not distort the signal. But that is a bit of a difficult way to spec it for the engineer designing it. The acceptance test is whether the software has bugs or not! lol
I think the reality is they don't even need the extra amplifier. There are two systems and I only see the amplifier board in the one, not the other that they wish to get in production. That's how poor the documentation is, we can't tell what the system is made of.
Speed or RPM sensing sensing North pole turns on south pole turns off (Honeywell SS361). Position sensing from two small magnets (electric windows or soft tops) etc....
In this case they were trying to use it as the power switch to supply
3V3 to ALL devices as in the actual 3V3 not an enable or anything like that.
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Paul Carpenter | paul@pcserviceselectronics.co.uk
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OK explain to me how you would fit a 48 pin QFP with 6 x 6 mm pin tip to pin tip dimension on DOUBLE sided board 8mm wide. When an area roughly 3 x 3 mmm underneath the chip must NOT be used must be ground plane. This is to avoid edges on the Digital signals interfering with the die, its ground and the bluetooth transmitter.
Also once you have left at minimum a 0.5 mm gap between copper and PCB edge you now have a MAXIMUM of 7mm PCB width to fit it in.
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Consider setting out special terms for contract but I would have required money up front and double rates.
Then when told we have not built a prototype yet, this is it, not even eval boards hooked together, I decided fly by night no idea merchants. No doubt they were later hoping for kickstarter to fund production.
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Hardly! This was developed by the government with no written use cases or any other project management goals. They had two different systems for two different applications (one which required more gain and so the extra amplifier board) and no "customers" asking for the system with the amplifier board. The first system works well enough that it is being used in both military and civilian applications as we speak. They need more of them pumped out though.
Still not sufficient info to analyze the problem. How many of the pins are needed? Which directions?
I'm not saying I can do magic. I'm saying I would discuss this with the customer to see what they had in mind. Clearly they either aren't giving you the full picture and they have something in mind or they have no idea what they are doing and it is impossible. I wasn't in any of the conversations so I can't say which.
I'm just saying I like tough problems and exploring alternatives.
BTW, what is the basis of the 0.5 mm gap around the perimeter? I know there are rules of thumb, but it is important to know the reasons for them so you can tell if they apply or not.
All pins in use, I had the circuit they wanted laid out. Just get a footptint including pad sizes and sketch it out to see what room you have.
You have 47 out of 48 pins in use just sketch it out with pin pitch of
0.4mm see how little space you have
Pretty standard PCB manufacture guideline of avoiding copper at edge of board for when final PCB is cut down to final size to avoid either breaking a track at edge or shorting between layers. Often this is 1 to
2 mm gap of copper to outline. Look at any PCB you have to hand even with a ground plane it does not go to actual edge of PCB.
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Then on double sided board for 0.4mm pitch work out where you fit the vias on the board edge pins and what size they and the tracks must be to avoid shorts, maintain etching gaps/clearances and what process will be required to manufacture it.
Remember with pads to solder to and effectively MAX of 7mm available PCB width, then 3mm taken up with central ground plane area how you fit on each board edge side 12 vias to take tracks to other side from a QFP footprint with 47 out of 48 pins used.
You will need to maintain some form of track/via/pad minimum clearance.
Due to edges with main power and bluetooth pins really not wanting to be on board edges you then have the crystal and other I/O mainly digital down the board edges.
Bluetooth really needs to be on non-board edge orientation for shaping and antenna filters with min track length and isolation from other areas so PCB antenna is not in middle of everything else.
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I've dug into this and I don't see the vias being the impossible aspect of this design. I I found one PCB vendor who will make a 12 mil pad (0.3 mm) with a 4 mil hole (0.1 mm) which will fit with 4 mil space along a row of 0.4 mm pads. This fits within the 1 mm chip to edge dimension allowing some extra space for pad extension (can't evaluate that until I know more about the package) and edge clearance. Hole to edge is a concern, but with such small holes it may not be an issue.
Via in pad is another option but I'm not sure would offer much advantage over the vias along the edge other than providing more hole to board edge clearance.
A second vendor has the capability to make the vias, but their auto quote page didn't allow boards smaller than 12 mm in either direction. They might make it if I ask for a custom quote.
This was after spending 10 minutes searching on the web for PCB fab capabilities.
I certainly would not bid this as a standard design. I would likely bump the price by a factor of three (or more) to cover the extra work involved in making it all fit. But I'm sure you saw more than what you've describe here. I'm just addressing the issue of the 48QFP routing.
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