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   ----- Transcript of session follows -----
550 TechNet... User unknown

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Response to:
=================================================================
Is there a Standard amount of Test Point Coverage at the PCB Bare 
Board Supplier during Electrical Test in which an NRE setup and 
100% Electrical Test of PCB's was charged.

What is the chance of Escape associated with Electrical Test?

Thanks in advance for any help given.

[log in to unmask]
=================================================================
Nancy those are good questions. 
I would like to see all the responses to your questions. Although
I have been in the bare board manufacturing the and PCB design 
business for more that 25 years I can't answer your specific
questions. The following is just my engineering opinions. 

First and formost is you get what you specifically ask for and 
you pay for all of it.

If you specifically ask for electrical testing of a Bare PCB, you 
should contact the supplier to understand the limitations of their
board board tester prior to designing your PCB and understand the 
coverage vs cost for various guaranteed reliability levels of product.    
   

Assume that you want to test 100% of all unique printed circuit 
features on one finished bare PCB for all possible shorts and opens
conditions.  Certainly you want each bare board to pass this test. 

You must design the PCB such that the Bare board tester can reliably 
contact ALL ends of All discrete printed circuit voltage, ground and 
signal nets simultaneously.  This can be costly and would limit your
design density to the tester probe to probe minimum grid and maximum 
number of probes allowed by the test equipment. Any design that 
does not allow for this complete coverage leaves some % posibility of 
defect detection escapes.  The answer is one of statistics for the 
specific board design in question.  Each net has the risk of shorts and 
/ or opens along it's entire path. Which other nets it has a risk to 
short with is a function of the routing path that that specific net 
takes.  The amount of risk is a function of the length and spacing of 
each net to the other.  Net to net shorts risk sites go up as the 
length that nets run adjacent to each other on any metal plane level
in the bare board increases.   You can see that the % of coverage
and % of potential escapes is very dependant on the specific test and
tester that the bare board fabricator uses.  As the PCB is designed 
with thiner printed circuit lines the risk of pc net opens goes up.
As the PCB is designed with closer line to line spacings the risk of 
pc shorts goes up.  
 
Manufacturing test engineers for bare board products need to calculate 
the % coverage and % defect escapes vs the risk and cost factors for their 
business.  NRE will be charged for any unique software or hardware that 
is needed for testing your bare board.  The rest of the cost for testing
should be buried in the unit cost of the bare board. 

I know that I did not answer any of your specific questions but as I 
suggested, contact each bare board manufacturer to get the specifics 
on their definition of "100% Electrical Test".  

100% electrical test might mean 100% of all possible shorts and opens 
were  tested for on some statistically significant sample size of 
each lot of PCBs produced and shipped. The remainder of the PCBs in
the lot might have had optical inspection only.  

Bare board testing for net to net shorts is sometimes done at the assembler prior to component attachment processes but this would add cost that you 
(the customer) would like to eliminate. 

The major assumption you have to make with all bare board testing is 
that your bare board manufacturer has qualified their processes and 
product to be reliable after stress testing.  Stress testing must 
simulate the subsequent manufacturing and functional usage conditions 
that the bare board is subjected to in it's product life. Bare boards 
are not stress tested as part of the normal manufacturing process.  

If you don't specify the type and extent of testing you want on a PCB 
you will get only what the PCB manufacturer can afford and still be 
competitive in their industry.  Sorry if this is no help to you, but 
who said that  electronic packaging engineering is simple.