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Re: Tutorial about IPC-7531A


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

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