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Design >> Analog Design >> Design for manufacturability
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Message started by thomasross20 on Oct 3^rd, 2012, 9:25am

Title: Design for manufacturability
Post by thomasross20 on Oct 3^rd, 2012, 9:25am

OK so this is an easy one. If I (arbitrary example) design an amp and the spec says it must have +/-10mV offset max/min.. in order to pass the manufacturing process with good Cpk & Cp I basically want to cut those limits in half and design to +/-5mV (which includes process corners, temp, supply voltage and random variation thrown in)?

I ask because I know many who don't stick to such strict guidelines. In fact it could be a disaster if you over-design based on this criteria (increasing power & area). So really, what we all should be doing is taking the spec limits and cutting them in half?

Title: Re: Design for manufacturability
Post by Lex on Oct 4^th, 2012, 1:34am

The over design does not come from manufacturability solely. It is related to all kind of uncertainty, such as device modeling, environment modeling, simulator accuracy, specification creep (i think loos-electron coined this term), etc., etc..

I think the least costly method would be to add digital circuits to provide some tuning afterwards. Though it adds more work on verification...

Title: Re: Design for manufacturability
Post by loose-electron on Oct 4^th, 2012, 2:07am

Depends on the particular case and design.

I have seen devices which were tested and selectively binned.

1-5-10mV offsets, and the lower offset device sold for a higher price.

If you are selling a single product, as a general rule, you want to get over 95%
yield to be cost effective.

If thats the case, then if you run full corners
and Monte Carlo on the design, you need to be better than 95%
of the time within desired specification.

Title: Re: Design for manufacturability
Post by thomasross20 on Oct 6^th, 2012, 1:31am

I agree, there are lots of uncertainties and you should always add a bit of 'sandbagging'... and yes, trim can help.

I know what you're saying, loose-electron. Sell the really good parts for more. If 95% are within spec limits then great. However is that enough? If all parts over process, temp etc etc are within those limits is that good enough? The product engineer is always banging on about getting good Cp and Cpk numbers, especially for chips designed for an automotive environment. So is it enough to be within spec, or must you design to half spec limits in order to meet the Cp & Cpk numbers.. I guess Cp and Cpk are used because the process can shift and you don't want it to sit outside your spec limits... but if we get a Cpk of less than 1.6 or so, I think the parts are binned - I must find out..