What a typical PCB experiences day in day out as far as temperature changes go.

 

Imagine this  modern day PCB motherboard, this is a multi-layer one which has numerous materials layered over each other, for example :

1. FR4 ( polyimide substrate)

2. copper foil laminate

3. solder masks

4. silk screen

5. ceramic substrate.

If this is your handphone or laptop PCs, there is daily temperature cycles going on when you power up your PC from sleep, or 'off' more. The temperature will go from (say) 23 C (inside the casing of your laptop) to maybe as much as 40 C in the course of the day.

When it is powered off, the temperature inside the laptop (PCB) will then slowly go back to 23 C in the evening.

Now, the key to this post is this. In the event, during the heatup phase, the temperature of the board rises. The  COEFFICIENTS OF THERMAL EXPANSION is different for all of the 5 different materials above, and hence some would expand faster than others, thereby pulling the neighbouring material.

When cooling down, the reverse effect is there, and hence there will be faster cooling of some materials than others.

When repeated hundreds of times a year (there are 365 days per year) then there is a chance than the incessant heat cool heat cool will result in BREAKAGES in the connection between the materials of the PCB motherboard, resulting in an open circuit and hence non functioning of the PCB motherboard.

Material selection is key and the QA and QC tests of temperature cycles is necessary to ensure that there are no catastrophic failures due to this repeated temperature cycles for at least 10,000 cycles or the expected lifespan of the laptop (say 5 years).

It is advised to do selective temperature cycling of PCB motherboard (powered up no less) in Test Labs or in house BEFORE the boards are shipped out to the next partner in the electronic supply chain to ensure 

> Quality

> Reliability

of the product is upheld.