You shouldn't have any problems with the axial leaded
components, providing you followed the bend radius
requirements.  The component, if elevated, will bend above the
board rather than at the pad.  You may have some problems with
your radial components if you only have 2-3 lead diameters from
the hole to the pad.  You could mount those components flush to
the board but may get some entrapment if coated.  We have, in
the past, stood the components off the board .015 and put the
strain releif on the bottom of the board where it attached to the
pad.

Larry Campbell
BFGoodrich, Avionics Systems

>>> Darrell Drake <[log in to unmask]> 05/20/98
10:50am >>>
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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