What are 6.5 Digit Multimeters Good For?
in [Design]
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From: Archimedes' Lever on 17 Mar 2010 23:12 On Wed, 17 Mar 2010 08:36:49 -0500, "Randomly" <fburfell2000(a)n_o_s_p_a_m.n_o_s_p_a_m.yahoo.com> wrote: >>On Tue, 16 Mar 2010 09:40:50 -0500, "Randomly" = >><fburfell2000(a)n_o_s_p_a_m.n_o_s_p_a_m.yahoo.com> wrote: >> >>> >>>>>You may be making measurement errors when you measure the traces, 1 = >>amp >>>can >>>>>be a lot of current on small traces and you may be heating that trace >= >>up >>>>>quite a bit. Copper has a substantial thermal coefficient of = >>resistance, >>>>>heat it up by 25C and the resistance goes up 10%. Try dropping your >>>current >>>>>to 0.1A and see how your measurements change. >>>> >>>>I think my measurements are accurate, and I often include a test trace >>>>whose geometry is suitable for accurate sheet resistance measurement. >>>> >>>>If I include a fab note demanding a minimum sheet resistance, or say >>>>"START WITH 1 OZ COPPER" I usually get below 600 uohms/square. If I >>>>just say "COPPERCLAD 1 OZ" I usually don't. >>> >>>Well if you are sure you are not heating your traces up when you measure >>>them it sounds like you need to talk to your fab about the issue. Get = >>them >>>to explain their fabrication process and what the variables are that can >>>affect the sheet resistance. 1 oz copper is just an arbitrary standard, >>>what's important is to know what sheet resistance you can expect out of >>>them and how variable it can be. It's good to know what the sheet >>>resistances and dielectric constants of all the board options they >offer= >> so >>>you can get what you need out of them. If it's an important aspect of = >>your >>>board design you have to specify it, you can't leave it to chance since >>>they may change suppliers, or processing, or subcontract it out etc. = >>Talk >>>to them, find out what the options are and what they are comfortable = >>with. >>>If you are not satisfied find another fab. >>> >>>What fab are you using? =20 >>> =09 >>>--------------------------------------- =09 >>>Posted through http://www.Electronics-Related.com >> >>Damn you do not read very well do you? JL stated that he orders boards = >>by=20 >>sheet resistivity (al least when he thinks it matters, did you pick up >on= >>=20 >>the test trace?). > >You've lost me. What did I say that was not valid? Perhaps my past >experiences with cheap chinese pcb manufacturers has tainted my outlook but >I am missing what point you are trying to make. > >--------------------------------------- >Posted through http://www.Electronics-Related.com He cries because some of his "engineering" involves relying directly on a specific length of trace to provide a specific micro-ohm resistance to make his circuit function in a repeatable manner, which is important. Though not likely the best way to attack the problem
From: John Larkin on 18 Mar 2010 12:51 On Wed, 17 Mar 2010 20:12:16 -0700, Archimedes' Lever <OneBigLever(a)InfiniteSeries.Org> wrote: >On Wed, 17 Mar 2010 08:36:49 -0500, "Randomly" ><fburfell2000(a)n_o_s_p_a_m.n_o_s_p_a_m.yahoo.com> wrote: > >>>On Tue, 16 Mar 2010 09:40:50 -0500, "Randomly" = >>><fburfell2000(a)n_o_s_p_a_m.n_o_s_p_a_m.yahoo.com> wrote: >>> >>>> >>>>>>You may be making measurement errors when you measure the traces, 1 = >>>amp >>>>can >>>>>>be a lot of current on small traces and you may be heating that trace >>= >>>up >>>>>>quite a bit. Copper has a substantial thermal coefficient of = >>>resistance, >>>>>>heat it up by 25C and the resistance goes up 10%. Try dropping your >>>>current >>>>>>to 0.1A and see how your measurements change. >>>>> >>>>>I think my measurements are accurate, and I often include a test trace >>>>>whose geometry is suitable for accurate sheet resistance measurement. >>>>> >>>>>If I include a fab note demanding a minimum sheet resistance, or say >>>>>"START WITH 1 OZ COPPER" I usually get below 600 uohms/square. If I >>>>>just say "COPPERCLAD 1 OZ" I usually don't. >>>> >>>>Well if you are sure you are not heating your traces up when you measure >>>>them it sounds like you need to talk to your fab about the issue. Get = >>>them >>>>to explain their fabrication process and what the variables are that can >>>>affect the sheet resistance. 1 oz copper is just an arbitrary standard, >>>>what's important is to know what sheet resistance you can expect out of >>>>them and how variable it can be. It's good to know what the sheet >>>>resistances and dielectric constants of all the board options they >>offer= >>> so >>>>you can get what you need out of them. If it's an important aspect of = >>>your >>>>board design you have to specify it, you can't leave it to chance since >>>>they may change suppliers, or processing, or subcontract it out etc. = >>>Talk >>>>to them, find out what the options are and what they are comfortable = >>>with. >>>>If you are not satisfied find another fab. >>>> >>>>What fab are you using? =20 >>>> =09 >>>>--------------------------------------- =09 >>>>Posted through http://www.Electronics-Related.com >>> >>>Damn you do not read very well do you? JL stated that he orders boards = >>>by=20 >>>sheet resistivity (al least when he thinks it matters, did you pick up >>on= >>>=20 >>>the test trace?). >> >>You've lost me. What did I say that was not valid? Perhaps my past >>experiences with cheap chinese pcb manufacturers has tainted my outlook but >>I am missing what point you are trying to make. >> >>--------------------------------------- >>Posted through http://www.Electronics-Related.com > > > He cries because some of his "engineering" involves relying directly on >a specific length of trace to provide a specific micro-ohm resistance to >make his circuit function in a repeatable manner, which is important. Not so. I have toyed with the idea of using pcb traces as current shunts and such, but I've never done it. The only time trace resistance really matters to me is when I do high-current stuff, where lower is always better. John
From: Fred Bartoli on 18 Mar 2010 13:01 John Larkin a �crit : > On Wed, 17 Mar 2010 20:12:16 -0700, Archimedes' Lever > <OneBigLever(a)InfiniteSeries.Org> wrote: > >> On Wed, 17 Mar 2010 08:36:49 -0500, "Randomly" >> <fburfell2000(a)n_o_s_p_a_m.n_o_s_p_a_m.yahoo.com> wrote: >> >>>> On Tue, 16 Mar 2010 09:40:50 -0500, "Randomly" = >>>> <fburfell2000(a)n_o_s_p_a_m.n_o_s_p_a_m.yahoo.com> wrote: >>>> >>>>>>> You may be making measurement errors when you measure the traces, 1 = >>>> amp >>>>> can >>>>>>> be a lot of current on small traces and you may be heating that trace >>> = >>>> up >>>>>>> quite a bit. Copper has a substantial thermal coefficient of = >>>> resistance, >>>>>>> heat it up by 25C and the resistance goes up 10%. Try dropping your >>>>> current >>>>>>> to 0.1A and see how your measurements change. >>>>>> I think my measurements are accurate, and I often include a test trace >>>>>> whose geometry is suitable for accurate sheet resistance measurement. >>>>>> >>>>>> If I include a fab note demanding a minimum sheet resistance, or say >>>>>> "START WITH 1 OZ COPPER" I usually get below 600 uohms/square. If I >>>>>> just say "COPPERCLAD 1 OZ" I usually don't. >>>>> Well if you are sure you are not heating your traces up when you measure >>>>> them it sounds like you need to talk to your fab about the issue. Get = >>>> them >>>>> to explain their fabrication process and what the variables are that can >>>>> affect the sheet resistance. 1 oz copper is just an arbitrary standard, >>>>> what's important is to know what sheet resistance you can expect out of >>>>> them and how variable it can be. It's good to know what the sheet >>>>> resistances and dielectric constants of all the board options they >>> offer= >>>> so >>>>> you can get what you need out of them. If it's an important aspect of = >>>> your >>>>> board design you have to specify it, you can't leave it to chance since >>>>> they may change suppliers, or processing, or subcontract it out etc. = >>>> Talk >>>>> to them, find out what the options are and what they are comfortable = >>>> with. >>>>> If you are not satisfied find another fab. >>>>> >>>>> What fab are you using? =20 >>>>> =09 >>>>> --------------------------------------- =09 >>>>> Posted through http://www.Electronics-Related.com >>>> Damn you do not read very well do you? JL stated that he orders boards = >>>> by=20 >>>> sheet resistivity (al least when he thinks it matters, did you pick up >>> on= >>>> =20 >>>> the test trace?). >>> You've lost me. What did I say that was not valid? Perhaps my past >>> experiences with cheap chinese pcb manufacturers has tainted my outlook but >>> I am missing what point you are trying to make. >>> >>> --------------------------------------- >>> Posted through http://www.Electronics-Related.com >> >> He cries because some of his "engineering" involves relying directly on >> a specific length of trace to provide a specific micro-ohm resistance to >> make his circuit function in a repeatable manner, which is important. > > > Not so. I have toyed with the idea of using pcb traces as current > shunts and such, but I've never done it. Did that once. I needed a very low inductance shunt. About 0.5"x1" for a 1mR shunt above a GND plane. Two vias and you have a four wire one. Worked like a charm, but that was for a current compensating loop, so I was servoing its voltage drop to zero and its actual value didn't matter much. -- Thanks, Fred.
From: Archimedes' Lever on 18 Mar 2010 20:31
On Thu, 18 Mar 2010 09:51:48 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Wed, 17 Mar 2010 20:12:16 -0700, Archimedes' Lever ><OneBigLever(a)InfiniteSeries.Org> wrote: > >>On Wed, 17 Mar 2010 08:36:49 -0500, "Randomly" >><fburfell2000(a)n_o_s_p_a_m.n_o_s_p_a_m.yahoo.com> wrote: >> >>>>On Tue, 16 Mar 2010 09:40:50 -0500, "Randomly" = >>>><fburfell2000(a)n_o_s_p_a_m.n_o_s_p_a_m.yahoo.com> wrote: >>>> >>>>> >>>>>>>You may be making measurement errors when you measure the traces, 1 = >>>>amp >>>>>can >>>>>>>be a lot of current on small traces and you may be heating that trace >>>= >>>>up >>>>>>>quite a bit. Copper has a substantial thermal coefficient of = >>>>resistance, >>>>>>>heat it up by 25C and the resistance goes up 10%. Try dropping your >>>>>current >>>>>>>to 0.1A and see how your measurements change. >>>>>> >>>>>>I think my measurements are accurate, and I often include a test trace >>>>>>whose geometry is suitable for accurate sheet resistance measurement. >>>>>> >>>>>>If I include a fab note demanding a minimum sheet resistance, or say >>>>>>"START WITH 1 OZ COPPER" I usually get below 600 uohms/square. If I >>>>>>just say "COPPERCLAD 1 OZ" I usually don't. >>>>> >>>>>Well if you are sure you are not heating your traces up when you measure >>>>>them it sounds like you need to talk to your fab about the issue. Get = >>>>them >>>>>to explain their fabrication process and what the variables are that can >>>>>affect the sheet resistance. 1 oz copper is just an arbitrary standard, >>>>>what's important is to know what sheet resistance you can expect out of >>>>>them and how variable it can be. It's good to know what the sheet >>>>>resistances and dielectric constants of all the board options they >>>offer= >>>> so >>>>>you can get what you need out of them. If it's an important aspect of = >>>>your >>>>>board design you have to specify it, you can't leave it to chance since >>>>>they may change suppliers, or processing, or subcontract it out etc. = >>>>Talk >>>>>to them, find out what the options are and what they are comfortable = >>>>with. >>>>>If you are not satisfied find another fab. >>>>> >>>>>What fab are you using? =20 >>>>> =09 >>>>>--------------------------------------- =09 >>>>>Posted through http://www.Electronics-Related.com >>>> >>>>Damn you do not read very well do you? JL stated that he orders boards = >>>>by=20 >>>>sheet resistivity (al least when he thinks it matters, did you pick up >>>on= >>>>=20 >>>>the test trace?). >>> >>>You've lost me. What did I say that was not valid? Perhaps my past >>>experiences with cheap chinese pcb manufacturers has tainted my outlook but >>>I am missing what point you are trying to make. >>> >>>--------------------------------------- >>>Posted through http://www.Electronics-Related.com >> >> >> He cries because some of his "engineering" involves relying directly on >>a specific length of trace to provide a specific micro-ohm resistance to >>make his circuit function in a repeatable manner, which is important. > > >Not so. I have toyed with the idea of using pcb traces as current >shunts and such, but I've never done it. The only time trace >resistance really matters to me is when I do high-current stuff, where >lower is always better. > >John Any of our traces where we were concerned about current ended up with multiple vias tying nodes together, and used both top and bottom sides when the circuitry runs allowed for it. Of course, I made power supplies. We had to know how to get from point A to point B. You guess as you go, and call it golden when it works, whether it is right or not. |