Your question on which direction is best implies that you may already question the value of HASS for circuit card testing. My article in this months issue of COTS Journal quantifies the lack of control that makes HASS screening HIGHLY questionalble for circuit cards. In stress screening where the goal is do enough damage to find production faults but not enough damage to cause failures in good hardware, it is a tough process when damage control can be off by many thousands (test to test, position to position).
In answer to your question, vibration perpendicular to the board is the best direction for thru-hole, surface mount and combinations. Occasionally, if the assembly has components at risk that are dominated by cross axis vibration - those could be added.
These principles are presented in CirVibe's course, www.CirVibe.com,on vibration life of electronics.
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