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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