Rich,
A clam shell tester IS a bed of nails tester except
a traditional bed of nails tester was only single sided.
A clam shell can test both sides simultaneously.
Then there is the flying probe tester, which only test
one or two point on both sides at a time.

The type of test you request determines what type of
tester is used. Requesting only a continuity test,
a flying probe tester is used (also low volume).
Requesting continuity & shorts & impedance (resistance)
tests requires the bed of nails or clam shell test.

This is why;

Clam shell is made of production/or volume and is in
contact will almost all of the contact point at the same
time allowing the tester to apply power to one pin/pad
and checking the destination that it did receive voltage.

While also checking lines that are parallel to the tested trace.
This provides conformation that the lines are not shorted. The shorting test
is forgone by some manufacturers to speed up the
testing process.

Flying probe is quicker to set-up for the manufacturer, but
the test process is much much slower. (reasoning behind
prototype testing).

A little note while were on the subject. If you request
electrical test specify the following points;
Continuity test
Shorting test (may want to specify Mohm(s))
Resistance test (may want to specify 5ohm per inch)

At the bare minimum, ensure what the resistance test is
capable of. I've experienced some board, and have
heard other note high resistance in connections due to
poor hole plating/cleaning.

This is also a good troubleshooting tip. If you are having
unexpected voltages, check the trace resistance.

Chris Robertson
[log in to unmask] <mailto:[log in to unmask]>

Author "PCB Designer's Reference"
to be released in Spring 2002







> What is the difference between bed of nails testing, clam
> shell testing,
> and SMT testing?
> I have a basic board that is ready for fabrication.  It is mostly
> surface mount with some through holes.  It has both analog and digital
> signals and no high speed.  Which test is best or least
> costly for this
> type of board?
>
>
> --
> Rich Klecka,  C.I.D.
> IPC Certified Interconnect Designer
> Electronic Packaging Specialist
> Fermilab
> P.O. Box 500,  M.S. 307
> Kirk & Wilson Roads
> Batavia, IL 60510
> [log in to unmask]
> 630-840-3880 Phone
> 630-840-8590 Fax
> Work hours: 8:00 - 4:30 Central Time
>
> --------------------------------------------------------------
> -------------------
> DesignerCouncil Mail List provided as a free service by IPC
> using LISTSERV 1.8d
> To unsubscribe, send a message to [log in to unmask] with
> following text in
> the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
> To set a vacation stop for delivery of DesignerCouncil send:
> SET DesignerCouncil NOMAIL
> Search previous postings at: www.ipc.org > On-Line Resources
> & Databases > E-mail Archives
> Please visit IPC web site (http://www.ipc.org/html/forum.htm)
> for additional
> information, or contact Keach Sasamori at [log in to unmask] or
> 847-509-9700 ext.5315
> --------------------------------------------------------------
> -------------------

---------------------------------------------------------------------------------
DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
To set a vacation stop for delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------