John, many thanks for the plug. Your comments regarding flying head 
versus bed of nails electrical testing are accurate. 

The main advantages of flying head testing are its ability to test 
very fine pitch geometries without probes interfering wit each other, 
and the fact that no fixture is needed to test the boards. It works 
well for low volume high technology testing.

Indeed flying head testing is slower than using fixtures with needle 
probes. Depending on the number of nets and the total number of 
points, it can take anywhere from 5 minutes to an hour to test a 
single board. To overcome this on large and/or complex lots, we 
identify a known good board with the flying head system, and then 
test the remainder on a bed of nails tester, by self-learning the 
golden board, and comparing the rest to the netlist the fixture based 
tester learned from the golden board.

Another area of concern is that of potential damage to small size 
pads, the force exerted by the moving probes when contacting small 
features can produce undesirable "scratch marks" if not set up 
properly.

It is worth mentioning that moving probe data preparation works based 
on extracting a netlist from Gerber data, and that it includes a 
method called adjacency, in which only nets that are physically close 
to each other are checked for isolation. Verification for opens can 
be set up to either test end-points only or to include intermediary 
points only, the former takes less time, the latter takes longer.

At CompuRoute, we compare the Gerber extracted netlist to the CAD 
design netlist prior to testing, to ensure that it is correct and not 
corrupted.

Like John mentioned, there is a lot more. Anyone interested in more 
information is welcome to contact me directly.

Luis Rivera
CompuRoute,Inc.
Dallas, TX
[log in to unmask]
(214) 340-0543

> From: [log in to unmask]
> Date: Thu, 12 Dec 1996 16:29:46 -0600
> Subject: Re: test: FLYING PROBE VS. HARD TOOLING
> To: Jim Marsico 516-595-5879 <[log in to unmask]>, [log in to unmask]
> 
>      Jim,
>      
>      There's more to this but I'll be brief.
>      
>      Basically both tests are the same when it comes to the software, you 
>      need a Netlist Test to effectively test the bareboard.  
>      
>      We used a Flying Probe Tester at previous employer, I think it was 
>      called a Probot (the model escapes me).  The basic Flying Head Tester 
>      is vertical with two flying test heads on either side of the 
>      bareboard, 4 total.  Two independent probes will make contact to where 
>      a NET begins and ends, if it be on one side or both.
>      
>      One big advantage of using the Flying Head Tester, it allows you to 
>      test non-grid layouts and grid layouts tighter than 20mils, very fine 
>      pitch devices (feature size) or basicallly tight densities that are 
>      outside the Bed-of-Nails capabilities (even if Clam Shell Testing is 
>      used).  The Flying Head Testers are used extensively with ATE cards 
>      specific to the Semiconductor industry that have tight DUT areas.
>      
>      The big disadvantage of a Flying Head Tester is TIME.  If you have 
>      2500 NETS, you have to individually test each net wheras Bed-of-Nails 
>      testing allows for complete coverage of the bareboard via test nails, 
>      unless you do a split Clam-shell test (Bed-of Nails but both sides and 
>      two separate fixtures).
>      
>      I'm not positively sure, but the cost for a Flying Head Test Unit is 
>      much cheaper than a Bed of Nails Unit and you do not have to pay 8 to 
>      10K for a Test Fixture each time a major revision occurs.
>      
>      Someone whom I would recommend and consider very knowledgable on the 
>      PROs and CONs of the Flying Probe Tester is Luis Rivera 
>      ([log in to unmask]).  He's extremely busy but is very helpful. 
>      
>      John Gulley - QA
>      INET Inc.
>      1255 W. 15th St. Ste 600
>      Plano, TX 
>      
> 
> ______________________________ Reply Separator _________________________________
> Subject: test: FLYING PROBE VS. HARD TOOLING
> Author:  Jim Marsico 516-595-5879 <[log in to unmask]> at Internet
> Date:    12/12/96 3:09 PM
> 
> 
> Helloooo, again....
>      
> Could anyone explain the difference between flying probe PWB testing and a hard 
> tooled bed-of-nails test?  Pros and Cons?
>      
> Thanks,
>      
> Jim Marsico
> (516) 595-5879
> [log in to unmask]
>      
 

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