Hi All, A problem with bed of nails testers is firstly the generation of the fixture & secondly inter-pad spacing due to the barrel size of the 'nail'. A flying probe is driven by the pcb data & hence has minimal setup & negligible inter head interference problems.
Cheers Don ----- Original Message ----- From: "Dennis Saputelli" <[EMAIL PROTECTED]> To: "Protel EDA Forum" <[EMAIL PROTECTED]> Sent: Thursday, September 26, 2002 1:59 PM Subject: Re: [PEDA] OT - bd testing > jeez Terry - that's a great response, thanks > > so is it fair to say that a bed 'o nails would do better at finding > random shorts or is it still more dependent on the software and test > programming? > > BTW, i've seen the little 'nicks' on the SMD pads, that is a good > telltale sign, but i always assumed that they were from the nails and > not a flying probe > but i don't know for sure > > i had no idea about the impedance test as an indication of the veracity > of the connectivity, that makes sense though, i guess this is indeed a > wonderful world > > so now that this aspect (impedance) is raised does that mean that as > part of a bare bd test if impedance control is required that it could or > would be specifically tested? > > i always thought that that was 'as designed' > i.e., if specific traces were called out as 50 Ohms or whatever that > as a part of a 'normal' bare board test that they would be verified as > well? > > or would that be considered an additional test call out ? > > Dennis Saputelli > > > Terry Harris wrote: > > > > On Wed, 25 Sep 2002 16:58:42 -0700, Dennis Saputelli wrote: > > > > >so how does a 'flying probe' test really work? > > >i understand the general idea of a couple of probes walking around the > > >comparing connectivity to a 'netlist' made from the gerbers > > > > >but it seems to me and i think i read somewhere that this is better at > > >finding opens than shorts > > > > I know the guy who runs a flying probe tester at my board shop. > > > > Depends, checking continuity between all 'terminal' nodes of a net is > > pretty easy. > > > > Testing for shorts between a net and every other net on the PCB would take > > forever. The test preparation software runs an adjacency algorithm trying > > to identify which other nets a particular net might be shorted to. This > > software hasn't been perfect and they had at least one instance of shipping > > boards with an undetected short. > > > > For boards with planes the tester they have can do a high frequency > > impedance analysis against the planes. It continuity checks one PCB to > > ensure it is 'gold' and measures 'something' at high frequency (I guess a > > MHz or so) with a single probe on each net. Subsequent boards are mostly > > tested using the impedance test, a single probe on each net being way > > faster. They find this testing method pretty reliable. > > > > If you are worried about your boards not been tested, if they are surface > > mount you can look for tiny holes left in the pads from the probes. I don't > > know if "bed of nails" testers leave the same indication. > > > > They recently got an optical inspection system which is very impressive. It > > tests almost as fast as you can load boards. It does actually inspect > > against gerber data with a rather complicated rule system for what is > > acceptable. It presents anything dubious to the operator on a video > > display. It was impressive to see the tiny nicks in tracks or bits of > > copper or dirt it picked up. It's like a manual inspection with a > > microscope but 1000 times faster and doesn't miss anything. > > > > They got it especially for inspecting the inner layers of multilayers (to > > avoid the waste of putting a faulty layer though subsequent processing) but > > it is so fast and effective they now seem to put all but the most basic > > jobs through it. > > > > Cheers, Terry. > > -- > ___________________________________________________________________________ > www.integratedcontrolsinc.com Integrated Controls, Inc. > tel: 415-647-0480 2851 21st Street > fax: 415-647-3003 San Francisco, CA 94110 > * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
