My responses are below, preceeded by *** JaMi Smith Optical Crossing Inc.
-----Original Message----- From: Abd ul-Rahman Lomax [mailto:[EMAIL PROTECTED]] Sent: Wednesday, January 23, 2002 9:57 AM To: Protel EDA Forum Cc: JaMi Smith Subject: Re: [PEDA] Looking for a resistor supplier. At 06:04 PM 1/22/2002 -0800, JaMi Smith wrote: >Now you really are looking for problems in your design! > >You should NEVER include solder plate in your CURRENT calculations [...] this is correct, but I'm not quite sure why the comment about "now you are really looking for problems...." No one suggested considering solder plating, as far as I noticed. Certainly I didn't. However, there is no harm considering solder plating, provided that one uses the correct values for its resistance. I'd expect, however, that it would be not worth the effort. *** I included it as a conversation starter, because, yes, you did infact include it in your comment, almost it seemed to me as if you had not realized it, which is exactly why it should be discussed in this forum, since it is something that so very often is overlooked. >, >since >Standard 60/40 solder only carries about 16% the amount of current as >copper, and that 16% is a very rough number that actually varies quit a >bit itself. Sure, but there are also plenty of other variables; if one is making an estimate, one might as well toss in a correction for solder plating, if there is to be any. Either (a) the resistance will be trimmed anyway or (b) there will be software compensation so the actual value is not critical. In this latter case, one is simply aiming for a reasonable value. *** #1, an unintended error of 8% far exceeds what any good designer wants to throw into any equation. #2, there is no known correction for an unknown variable, i.e.: I obtained the 16% number years ago from a scientist at Multicore, and he said that it was good for a specific composition of solder only, and that other solder compositions could vary up to an additional 5%. ALTHOUGH REALISTIC, THE 16% NUMBER IS ONLY APPROXIMATE AND SERVES TO SHOW EXACTLY WHY THE SOLDER SHOULD BE EXCLUDED FROM THE EQUATION - PRECISELY BECAUSE IT IS SO MUCH LESS THAN COPPER AND ALSO BECAUSE IT IS SO VARIABLE BY ITSELF. #3, Why on earth do you want to complicate the problem by adding a process that is the equivalent to an old SELECT IN TEST procedure to an otherwise simple design that you are going to produce 100 to 200 of (if I remember correctly), a procedure which by itself would cost as much in technician time man hours as buying a whole reel of 1800 parts for each ot the 100 to 200 boards would. However, the solder plating will not make a huge difference, so an urgent warning to avoid including it is a bit overstated. *** Assuming 1/2 ounce copper and the same additional thickness in solder, would yield you a home made "sense resistor" with the value only about 58% of what you would have thought you would have had if you included the solder in the equation and calculated for a total thickness of 1 oz. "a bit overstated" ??????????? I don't think you quite understand the scope of the problem . . . >Virtually ALL of the CURRENT calculators or tables or graphs which >determine the amount temperature rise above ambient for a given width >and thickness of copper trace, are ALL based on the old MIL STD 275 >Graphs which specifically tell you NOT to include the solder thickness >in the calculations for precisely this reason. I would not use the term "reason" in respect to those old tables. I've never been convinced that they were soundly based, I've never seen experimental verification. Are the tables conservative (i.e., actual temperature rise will be expected to be *lower* than stated) or are they predictive (i.e., one would expect experimental evidence of temperature rise to be approximately what is given in the tables)? *** You are obviously not either an electronics technician nor an engineer. I let my commenting stop at that poinr, and simply say that you should ask an engineer about the problem. >CALCULATE THE COPPER THICKNESS ONLY! > >Any calculations to determine the drop in milliohms for a given width, >thickness, and length of copper trace, should likewise not include >solder, for precisely the same reason. No, this is not true. One should not use solder plating thickness for the temperature rise tables because they were designed not to use it. There is *no* harm in compensating for solder plating thickness; however, one should know what variables are involved: reistance variations based on mixture, process variations in thickness, edge effects, etc. In fact, I'd recommend at least looking at the effect of the solder plating, because it *will* affect the final resistance, the only question is how much it will affect it. *** #1, Wrong! Go back and read NOTE 7 on the old MIL STD 275 "charts"! #2 If you could accurately determine what the exact characteristics of the specific solder that you were using you might begin to have an argueable point, except for 2 problems, the first of which is that you are only going to gain an additional 16% (approximate) in current carrying capacity, and you are forgetting the most important thing about solder, which is, that it melts, and that when the current gets high and the solder gets hot then at some point it will melt and reflow , which means that your calculated extra current carrying capacity just flowed away . . . . . #3, You have obviously never seen a copper trace get hot enough to burn off the solder plating on it, have you? i.e.: under certain circumstances your extra solder plating is going to go up in smoke, and when it does, I hope you weren't really counting on the solder to carry any extra current, because when the solder is gone, then your trace will most certainly become a fuse. More to the point, one may wish to make a board depending on trace resistance SMOBC (solder mask over bare copper), because the copper thickness and plating may be better controlled. Further, wider/longer traces would be preferred to thinner/shorter traces, because the process variations in trace width would have less of an effect. *** So . . . What has this got to do with whether or not to include solder in your current calculations? >In either case, you should not include solder plate in either the >calculations or the design itself. In other words, any solder plating >will screw up the actual values. This means that for this type of design >you would need to soldermask the specific trace in question, or >otherwise prevent it from being plated with solder. Obviously an unmasked trace, covered with solder, which might be quite thick, would be unsuitable for use as a controlled-resistance trace. But the norm would be that such a trace would be solder-masked anyway. Once again, the effect of having solder on the pads will be small if the trace is relatively long. *** So . . . If this is a board which is going to be produced in very small quantities, it may be more cost-effective to use a commercial part that would not need trimming, etc. Even in larger quantities, if trimming can be avoided by using a commercial resistor, and the resistor is inexpensive enough, it might be cheaper. Note, however, that simulating a Kelvin resistor on a circuit board would be easy, if the application requires that kind of accuracy. *** The part cost $11.80 for 10 from Digikey, that is monumentally cheaper than any of the other processes that you have discussed, and exponentially less complicated. For lowest cost, I'd go for software compensation.... *** Duh! . . . are you now going to include a micro in this design just to compensate for all of your other "design features" that you would now have us incorporate, or were you instead going to subject each bare board to a complex and expensive analysis performed by some super automated testing system which needed some test engineer to operate it and a week to program it. Hello! And for accuracy, one could include a temperature sensor sitting on the trace and temperature compensate as well. Or a test current periodically injected might be even better, it would deal with all kinds of variations in one fell swoop. *** Another $10.00 circuit just to watch the extras that you are going to add to the original design, and gee, a precision thermistor to boot, or maybe we could just use another "current sense" circuit to sense the current in the original "current sense" circuit, but then again, that might need another "current sense" resistor. *** The amount of time that has been wasted in this exchange is phenomenal, but I have persisted for the very same reason that I wrote the first response, which is that I believe that there are many people on this list that did not know that they should NEVER INCLUDE THE SOLDER PLATING IN THE CURRENT CALCULATIONS. After this exchange, they will never forget it. *** The bottom line is this, don't count the solder, and the minor benefit that you will gain from it will always be in your favor. *** For those of you that have the capability, go out to the lab and take a well protected variable supply and run some current thru a piece of solder, and see what it will do compared to a nice copper conductor of the same size. *** Knowledge, Logic, and Reason, have no substitutes. js * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 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://firstname.lastname@example.org * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *