This is an application where I would opt to install the components flush to
the board for the very machanaical reasons that you state in your original
message.  Failure to do so is to invite premature mechanical failure when
the PWA is placed into operation.  The lead forms to make the connections
required will provide more than enough lead compliance in most cases.  The
biggest thing that you have to watch for are individual components such as
power transistors, triacs, etc. that might lead to heat damage of the board
substrate.  These need to be properly supported and heat sinks used.

Dave Robertson <[log in to unmask]>
Chairman 5-22c Manual Soldedring Task Group

At 09:50 AM 5/20/98 -0500, you wrote:
>I am working with a customer's PCB design that uses non-plated
(unsupported) through holes and connects to a lap pad on the circuit side
(the pad is approx. 2 to 3  lead lengths from the hole). The components to
be mounted consist of both axial a radial passives (horizontal for
resistors, caps, etc., & vertical for transistors, etc). My question for
discussion and comment is whether to or not to mount the component body
flush to the substrate. If you elevate the component body off of the
substrate without some type of spacer or support, during handling it is
highly probable that a downward force applied to the component will result
in lifted/damaged pads on the opposite (circuit) side. I am familiar with
the use of  spacers/supports for supported holes (PTH) and that it is
desirable to maintain some spacing from the substrate for lead compliancy;
however, I have not found references which address the aforementioned. The
typical operating environments are high vibration coupled with high
temperature (>=150degC <=200degC). The PCB substrate is typically polyimide
and ranges in thickness from .062" to .090" .
>
>Any comments ? Expert opinions ?
>
>D. Drake
>
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--
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David E. Robertson     (908) 245-6200
Sales Manager           (908) 245-6176 fax
Hexacon Electric Co.
161 W. Clay Ave.
Roselle Park NJ 07204-1946

e-Mail: [log in to unmask]
  URL: http://www.hexaconelectric.com
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