Mail*Link(r) SMTP FWD>DES/FAB/ASSY: Ruggedized Revisited
Jeff,
Sorry, but in my previous response I forgot to include some
thoughts/experiences on component mounting on the PB itself.
In the past, we've "ruggedized" many commercial/industrial Class 2 products to
withstand the rigors of some of our R&D environments. Generally
shock/vibration is the major concern and therefore, the "ruggedization" of the
component mountings need to be considered for PBA's being subjected to shock
and vibration is a major concern.
Vertically Mounted Axial Leaded components:
Some of the concerns are vertically mounted, axial leaded components. The
greater the mass the greater the problem. Lay them flat against the PB if
possible, if not stake at least a couple (three preferred and hopefully in a
"triangular" layout) together using a non-corrosive RTV staking adhesive.
lllll
aaaaaaa aaaa l l
cccccc cccccc ccccc l where the "a"s are the staking adhesive
c c c c c c l
c c c c c c l the "c"s are the component body
c c c c c c l
c c c c c c l the "l"s are the component leads
cccccc cccccc ccccc l
l l l l l l the "pb"s are the printed board
pbpbpbpbpbpbpbpbpbpbpbpbpbpbpbpb
l l l l l l
It is best if the components can be bonded together to for a
tetrahedronal-like truss for "mutual" support.
Also, becareful with power semi's, especially the TO-220 like families, in
particular those that have clip/slide-on cooling sinks, use a cooling sink
that mounts into the PB.
In some cases, you have large "single" components that are mounted away from
everything else (isolated) and there is no way to obtain "mutual" support, in
these cases, sometimes it is possible to add a fastener mounted "standoff" (or
brace) along side the component and use either a staking adhesive to bond the
component to the standoff or to use a plastic cable tie-wrap(s). A couple of
times we needed to "ruggedize" normal Class 2 for limited-use automotive like
environments and there wasn't room to add "standoff's". So we adhesive bonded
3 diagonal braces from the top of the component to available open areas on the
PB, it didn't look great, but for R&D applications, it worked.
Horizontally Mounted Components
Observe the mass/component lead requirements in IPC's design and assembly
spec's. Most components up to a 2W resistor like size are not a problem.
Large electrolytic capacitors, radial or axial leaded can be a concer. Tie
them down or adhesive bond them.
Large power resistors and inductors are sometimes mounted 6mm and more off the
surface of the PB --- it's a disaster. For power resistors, it's done so as
not to burn holes in the PB base material. For inductors it to provide
cooling and to reduce the distributed capacitance across the inductor. Though
most of you aren't into high voltages (>a few hundred volts) it controls the
electrical spacing. For high impedance analog stuff, it reduces the
capacitance controls the insulation resistance for high valued feedback
resistors (Rf) [Rf -- that's not radio frequency that's electronickers
shorthand for the feedback resistor] in analog amplifiers.
These pose real problems, but you need to either mount them lower or provide
some kind of support for them. For example:
ccccccccccccccccccccccccc
--c c-- where the "c"s are the component body
l ccccccccccccccccccccccccc l
l ssss ssss l the "-"s & "l"s are the component leads
l ssss ssss l
l ssss ssss l the "s"s are low dielectric constant
plastic
l ssss ssss l spacers that are adhesive bonded to the
PB
pbpbpbpbpbpbpbpbpbpbpbpbpbpbpbpbpbpbpb and to the component body.
l l
the "pb"s are the printed board
Avoid the use of "plug-in" socketed components and connectors, or use one's
with shock/vibration restraints or use a staking adhesive to hold the mated
components in place using a non-corrosive staking adhesive (we genrally use
RTV's).
Hope this helps,
Ralph Hersey
e-mail [log in to unmask]
--------------------------------------
Date: 7/22/96 4:41 PM
From: Jeff Seeger
Hello, TechNet!
I have a situation where our customer is being asked for "ruggedized"
product. The product category is generally Type 3 and 4 PWB's, with
Class 2 expectations, fine pitch components both sides, (of course)
moderate high-speed rules. Product is built in ~100 piece lots.
Is there a general scheme of "ruggedizing" the PWA, or does this
simply imply an upgrade to Class 3 standards? I'm not sure density
will allow a full Class 3 implementation.
Is there anything non-subjective about "ruggedizing"? I've not seen
it mentioned anywhere.
Would selective upgrades to Class 3 qualify as "ruggedized"? Which
areas?
Of course, I have no specific performance requirements, just the
word "ruggedized". From the nature of the product, I would guess
there could be application for mobile equipment.
Can any "rugged" folks out help an old soft-in-the-head design guy?
Thanks alot, and best regards,
Jeff Seeger Applied CAD Knowledge Inc
Chief Technical Officer Tyngsboro, MA 01879
[log in to unmask] 508 649 9800
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Subject: DES/FAB/ASSY: Ruggedized PWA's
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