Once upon a time, there was a young QE, quality engineer. He had just
completed training the assembly and inspection staff to IPC-A-610B.
One of the newly trained inspectors brought a prototype speaker crossover to
him. The bodies of all the through hole power resistors (10 and 20 watt)
were in contact with the PWB, a very nice FR-4 board, intended to replace
the hand wired phenolic crossovers from the old days.
The QE brought the non-conformance to the attention of a newbie EE. The
Newbie took the position that the crossover was so over engineered that the
resistors wouldn't even get warm. (The Newbie knew that only a small percent
of the music power was in the band of the tweeter and mid range drivers. He
failed to realize that the crossover components do see the power as they are
providing the band pass.)
The QE responded, "Dude, we just spent thousands teaching the inspectors
right from wrong. Perhaps you are right, perhaps it doesn't matter, in this
case. Can we just run a little test? Let us rework one of the prototypes,
using a piece of 0.062" board scrap to set the thickness of the gap. Then
we'll install one of each type in a pair of speakers to see what happens."
They agreed to allow one of the young test technicians to select a music
program of his choice and play the CD through the test system at a sound
pressure level similar to what he might if mom and dad had left him home
alone for the evening.
Neither the QE nor the EE could tolerate the selection or SPL for long. They
left for 20 minutes and returned to a stinking smoke filled room.
Inside the cabinet with the IPC air gap: The pink fiberglass stuffing had
retreated, melted into little glass beads, leaving the resistors in a little
pocket that reminded one of wet cotton candy. There was no other evidence of
heat.
Inside the cabinet with the resistors contacting the board: The resistors
were no longer contacting the board, there was no board. Rather the
resistors were suspended on four ounce printed copper wire surrounded by
something that resembled burnt marshmallow.
IMO the Telcordia spec makes more sense, as a rule of thumb, for resistors
that dissipate heat and are intended to be cooled by air. If they are
designed for heat sinks or the design includes heat sink clips. Then you
cannot just use rule of thumb.
This really is a design problem. Proceed with caution.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Wednesday, May 14, 2008 9:06 AM
To: [log in to unmask]
Subject: [TN] Power components standoff
Hi Technos,
Is there any standard that states standoff height with respect to the PCB
for power components, like 1W and more resistors? The only number that
IPC-610 and J-STD-001 advance is minimum 1.5mm. Is there anything available
that relates power to height? I mean 5W comps should be higher than 1W, but
how high?
I am trying to standardize component prepping...
Thanks,
Ioan
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