If you had designed this rear cover, what are 5 tests that you would conduct on the completed iPod Touch assembly to ensure that your rear cover design met all applicable requirements? Why?

Mechanical Engineering Reliability & Mechanical Test
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Background: The interviewer set the rear cover of an old iPod Touch on the table and asked me to identify the materials. If you had designed this rear cover, what are 5 tests that you would conduct on the completed iPod Touch assembly to ensure that your rear cover design met all applicable requirements? Why?

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I would start by standardized drop, impact and transportation tests. Maybe a worst case temperature test and an Ingress Protection test.

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To answer the first point above, an easy way to distinguish materials commonly used on phones (i.e. aluminum or stainless steel) is to take a brass key and scratch the back cover. Brass has a higher Rockwell Hardness value than aluminum, but lower than stainless steel. An aluminum back casing will scratch in this case. Additionally, you’ll be able to roughly tell by the weight of the casing (stainless steel is about 2.5x denser than aluminum) and the luster of the metal (aluminum typically has a dull, visual appearance).

To approach this is, start first with the product requirements (PRD) which are documented out during the early stages of product inception. For example, the iPod Touch will have specified conditions regarding product lifetime, durability, rated environment conditions.

Working from that understanding you can start establishing your baseline cases. For example, for it to be rated for all aspects of general use, you would subject the rear cover to different HALT (highly accelerated life tests) through functional tests at set ambient temperature & humidity ranges. This is just one example.

Regarding durability, the PRD could specify a rated height for the enclosure to not deform or crack and this would be another standardized drop test to conduct.

Generally speaking, tests can be broken up into two categories in which one branch covers product architecture validation, and another covers reliability testing (i.e. testing to simulate spanning multiple years).

Here is a list of five tests that are important:

  1. High Temperature, High Humidity Test
  2. Short & Tall Drop Test
  3. IPx6 Waterproofing Test
  4. Chemical Exposure
  5. Abrasion Test