Retronix Blog

May 11, 2018

CTE Issues – Harsh Environment, Automotive, High reliability industries.


What is Coefficient of Thermal Expansion (CTE)?


Coefficient of Thermal Expansion (CTE) is a measure of the expansion or contraction of a material as a result of changes in temperature. Materials expand because an increase in temperature leads to greater thermal vibration of the atoms in a material, and hence to an increase in the average separation distance of adjacent atoms. The linear coefficient of thermal expansion ‘α’ (Greek letter alpha) describes by how much a material will expand for each degree of temperature increase.

Coefficient of thermal expansion affects automotive, harsh environment and high reliability electronics in particular.

The recommendation is that thermomechanical stresses due to different coefficient of thermal expansion in the materials used in electronic assembly should be factored in during the design phase. The issue with this is that this is not always possible due to the expansion rates not always being known.

Stress fractures can occur not just at the assembly stage, but when the assemblies are in their end use environment. The stress the PCB and components can be placed under can cause CTE to take place and fractures to show over a period of time due to the temperature and environment of the end use.

Fractures due to CTE issues

CTE puts the solder joint under stress since the two materials that the solder is joining are expanding at two different rates, so this makes joints that use a smaller amount of solder or a lower contact area most at risk of this fracturing. A mismatch in the materials expansion rate puts these joints under a stress that the surface area of the solder cannot absorb, therefore leading to fractures in the solder connection. This fracture may not be enough to always stop the functioning of the part straight away, but can lead to a slow degradation in the function of the assembly and an eventual critical failure.


A micro fracture in a solder joint on an assembly that is under use in a benign environment may never cause a critical failure, but if you factor in vibration and temperature changes that PCBs in the automotive industry for example would be under then a micro fracture can very soon become a critical failure.


Ceramic BGA’s are at a higher risk of CTE fractures due to a large thermal mismatch between the ceramic material and the PCB laminate. Ceramic packages are largely manufactured  for the high reliability market.

Plastic BGA’s have a closer matched thermal expansion rate to common PCB material due to the internal structure of the BGA normally being Bismaleimide-Triazine (BT) and this material is closely CTE matched to FR-4 and Polyimide laminates used in PCB manufacture.

BGAs with smaller sphere sizes are more susceptible to CTE fracturing, larger sphere sizes provide a better stand off and more mechanical compliance. The larger solder mass in the intermetallic connection also provides better thermal conductivity and ability to absorb the CTE issues.

This is what often forces high reliability and harsh environment electronics to use column grid arrays for ceramic packages. These devices are expensive and may not be a readily available option for the device that has been designed into the assembly.

What Retronix offer is a more cost effective, repeatable and reliable solution than adding columns to packages that have been manufactured and sold as a BGAs.  Retronix use their latest technology laser reballing system to add solder stacks to the pads on the BGA, the balls are stacked on top of each other to create a larger mass of solder and an increase in flexibility to counteract the CTE effect.

Standoff Retronix Solutions

Utilising the laser reballing system we can also add balls to the underside of QFNs, LGAs and any Bottom only Terminated Component. This has the same effect of assisting with CTE issues on these parts as the component will be placed with a larger solder mass and better thermal conductivity between the component and the PCB.

This is also a very effective solution for the voiding issues often seen on bottom only terminated components during reflow. 

Standoff Retronix Solution

 

If you would like to learn more about how Retronix can assist you with CTE issues you may be facing please get in touch with us via the Contact Form or email us at rob.ronan@retronix.com or sales@retronix.com


We are willing to undertake Free Of Charge sample trials to enable you to run environmental temperature cycles to see if our Service is your Solution


Original Article published on LinkedIn by Rob Ronan (UK Sales & Support Manager)

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