PWB testers like the IntegraTest machine we use measure the capacitance of a net to identify it. In this way it reduces the exhaustive test time to something doable. With a totally dumb machine you'd have to test every pad to every other pad. Typically the test begins with the continuity test where it's verified that each pad in a net has the same capacitance and has a minimum conductance between each pad. During the "shorts" test only those nets with approximately the same capacitance need to be resistance verified. John
Terry Harris wrote: > On Wed, 25 Sep 2002 20:59:10 -0700, Dennis Saputelli wrote: > > >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 called it an impedance test but I really don't know what it is doing. If > you just measured the capacitance between a net and a plane the vast > majority of opens and shorts would produce a significant difference. > > The tester only compares whatever it measures against the 'gold' board, it > is saying this board is like a good one, not measuring any particular > parameters. > > >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? > > Controlled (transmission line type) impedance is a who different can of > worms. I imagine others here know more about it than I do. > > I as far as I know testing is limited to test traces (usually outside the > PCB outline). You can buy trace impedance measuring equipment (which seems > very expensive for what it does) so the board house can measure your test > traces. The problems start when what you measure is not what you want. > > Firstly you can't measure it till the board is finished and if it is wrong > the board (and most likely the whole batch) is scrap. > > Secondly the trace impedance is mostly determined by the designers stack up > and trace widths, if the designer gave you something that is going to be > about 100 ohms there is no way the board house can make it 50 ohms (unless > you are happy to have your 1.6mm board come back as 1mm ?). > > The board house does have some control of prepreg and core thicknesses, > material dielectric constant, and control of trace widths through etching > and plating. > > The board house I use looked into offering controlled impedance boards and > decided against it for the time being. Apart from the expense of the test > equipment how do you cost a board that you might have to scrap and remake > three times? To be good at it they would have to gain a lot of experience > and put a lot more effort into material and process control than they need > to currently. They were also worried about customers ignorant of the > issues. > > Cheers, Terry. -- >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >John M. Cardone Electro-Mechanical Dsgn. Engr. Grp. >M/S 278-100 Mechanical Engineering Section, 352 >4800 Oak Grove Dr. NASA / Jet Propulsion Laboratory >Pasadena, Ca 91109 >Tel: 818.354.5407 MailTo:[EMAIL PROTECTED] >Fax: 818.393.6400 Cell: 818.653.7818 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 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/[email protected] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
