On Wed, 1 Sep 2010, Alex G wrote:
Thanks!!! This should prove very useful. I like their idea with the RMS sum for tolerances.
It's actually a rule from statistic... summing the whole tolerances is the `worst case' method. If (and only then) the tolerance have gaussian distribution (and since we're talking about manufacturing thats probably true) you can take their RSS (Root Squared Sum) instead, and still have a representative value. Google for 'summing tolerances' to get copious explanations. In other word, they simply applyed the usual rules :D
If I had the standard, I would quote the tables (with proper references of course). No, I'm not asking you to do it; I already have that info in LP Calculator. In fact, I'm not even suggesting it. It's just my way of saying I live on the dangerous edge of life. :-p
Also, they are biggish (look at the list of the pin type, each pin has a table for the three manufacturing densities with all the fillet targets and courtyard excess) and easy to misquote. Since they would be hand copied anyway I just offered a `reliable source'. And they copied the `round to 0.05, 0.10, 0.15 and so on' explanation in *each table* as `roundoff factor'. I'm looking in the rest of the standard for juicy bits... it's mostly stuff like: QFN should survive reflowing at least 5 time, blah blah, or what differ in etching if you use phototools vs laser engraving and liquid resist vs dry resist, fixturing for panelization and so on (i.e. very low level stuff for the people actually fabricating boards). Oh, and *vital* information like: Tape peel strength [they're talking about components reel] shall be 40 grams +- 30 grams (/me imagines a Q.A. technician measuring with a dynamometer the peeling strength of one reel :D:D) And of course there are IPC-7251 (i.e. through hole components, we need it to replicate the corresponding function in the LP Calculator) and IPC-2222 (mixed wisdom about overall PCB design, layer stacking and so on -- including test coupons in panels and using some little resistor clips if you need a board for the space shuttle :P).
I say you should include a schematic for a GPS guided missile so that someone may send a free copy of your work to whatever entity wrote the standard (non-destructively, of course). :-p
I suggest to send it to Mentor Graphics which acquired PCB Matrix (the developer of the calculator). Or, also, to IPC itself which in the back cover essentially says: 'You have to pay because we spent a lot of time researching it and partecipating meetings with a lot of red tape with ANSI and so on' They don't even offer an `upgrade license' for the owner of the old standard, you have to pay it fully... (you only have 50% off if you have *membership* with IPC, I don't dare to ask what they want each year:D) -- Lorenzo Marcantonio Logos Srl _______________________________________________ Mailing list: https://launchpad.net/~kicad-developers Post to : [email protected] Unsubscribe : https://launchpad.net/~kicad-developers More help : https://help.launchpad.net/ListHelp
