High Change Rate Temperature Cycling Tests for Printed Circuit Boards.

 

                            CTE mismatch (above) leads to elongation and contraction ;

     Over time, this mismatch will lead to crack and intermittent or catastrophic failure. 

This is the basis for High Change Rate Temperature Cycling Tests, which are typically conducted in a chamber called, Environmental Stress Screening Chamber. Stress Screening Chambers come either in Single Zone or Two Zone type. 

QRA is proud to market these ESS chambers under our brand name, and also under our principal from S Korea, ETSP. 

Lets go straight to the facts. Below is a diagram of a typical PCB board, which can be used in a myriad of applications from latest smart phones, notebooks, IPads and EV cars. The top part is the silicon chip (think Nvidia, Lenovo and Intel) which does all the microprocessing, and the bottom part connects the chip to the Printed Circuit Board. The connections are provided by solder bumps or balls and the point of contact is called the solder joints.

To improve transmission speed, the area in yellow is called the underfill and the solder bump sits on a contact pad.

The issue with reliability of PCB Boards, is that owing to cost pressures, the silicon chip is typically of a certain Coefficient of Thermal Expansion (CTE) and is normally on the low side, ranging from 2.5 - 4 ppm / C. The organic base substrate for cost reasons has a high CTE something like 20 - 30 ppm / C. 

When the device is powered up (smartphone, car or laptop), the underfill expands very fast and longer, while the silicon chip expands slower and at a smaller overall length.

When the device is powered down or switched off, the rate of contraction between the Organic Base Substrate and the Silicon Chip again is very wide, leading to the solder bumps positioning from the top picture to the bottom fixture. With repeated switch on and switch off, the solder bumps will crack over thousands of cycles. 

Therein lies the CTE Mismatch. Over time, this mismatch of elongation and contraction will lead to crack on the solder bump and the chip will be non - functioning. 

Its like a loose tooth, if you move it regularly, over time the gum base will give way and the tooth will fall out.  

The ESS chamber is simulating the power on and off for anything ranging from 100 cycles to up to 1000 cycles. There are standards which typify the environment which the boards are typically subjected to, to replicate the actual functioning. 

Set points can range from below 0 (-20 C) to about 100 C and the change rates vary from 1 C / min up to 20 C / min. 

                                       2  Zone ESS Chamber or Thermal Shock Chamber 

Why Do the Temperature Cycle Tests ? 

1. To ensure that the solder bump CTE is not too far off from the silicon chip, as well as the organic base substrate chosen (its all a matter of costs), is also not too high compared to the silicon chip.

2. If after 1000 cycles, the PCB Board is subjected to 

a) functional tests to ensure board functionality with a 'Pass / Fail' checking

b) High magnification Visual Inspection to determine microscopic cracks.

The reliability of PCB Boards, is critical to the high end brand's overall good name.

If after all the ESS tests are done, and the checks give a 95% yield (5% failure rate), then the batch of PCB Boards can be considered good for production and the assembly can proceed.

If the yields are far higher, (say 20 % upwards), the manufacturing manager1  may conduct an internal audit to review the selection choices of 

 

> Organic Base Substrate

> Solder Ball Materials.

Reliability is essentially Quality and Performance of a Product over the entire Life Time of the Product ( 2, 3 up to 10 years for EV cars).

Top manufacturers cannot compromise on poor out field reliability. 

It is better to recall and rework the new product in the factory before even a single batch is sent out. 

Manufacturers who have to conduct a product recall face a huge loss of brand confidence leading to huge revenue dips and profit  losses from a (perceived) poor reliability product. 

ESS Temperature Cycle Tests save millions if done properly. 

For more information ;

E :    mark@qra.com.sg

        qrasales@qra.com.sg

W :  https:\\www.qra.com.sg