In today’s electronics, solder joint cracks and fractures are becoming a more and more frequent issue as modules are used in harsher field conditions, are more densely populated with components and are more heat generating. As there are typically three root causes; short-term loading, creep and thermal mismatch, often the thermal mismatch is the main cause.
Thermal or CTE mismatch is the fatigue failure that result from cyclic loading and it depends on the difference in expansion coefficient of a material, the difference in temperature and the size of the component. Mathematically it is given by (1):
Du = De * L * DT
Du= Thermal mismatch
De=difference in CTE between the materials
L= longest dimension of the component (diagonal)
DT= temperature change
Fig 1: Illustration of vulnerability to solder joint cracks on a PCB – source (2)
thus in order to minimize solder joint cracks, it is necessary to minimize the thermal mismatch which can be done by decreasing the difference in CTE, minimizing the temperature delta or reducing the size of the component. As the component is often a given and as well as the field conditions , the remainder of this article focuses on decreasing the CTE mismatch.
CTE mismatch reduction
Let us take a look at the typical CTE values of common substrates and packages (in ppm/℃).
Table 1: typical CTE values of common substrate and component material
|Substrate / PCB material||CTE-value||Packages / Components||CTE-value|
|Aluminum||25||Silicon Carbide (SiC)||2.7-5|
|Copper foil||17||Alumina (Al2O3)||6-7|
|AluminaOxide (Al2O3)||6-7||Aluminum Nitride (AlN)||3-4|
|Aluminum Nitride (AlN)||3-4||Gallium Nitride (GaN)||5.4-7.2|
|CEM-3||24||Gallium arsenide (GaAs)||5-6|
Noticeably, many of the components have CTE values in the single digits whereas PCB substrate materials are often above except for the ceramic materials; Alumina and Aluminum Nitride. Also copper-invar-copper (CIC) laminates are often below 10.
To illustrate the difference in stress we take the example of a power LED. Assume an automotive application where field conditions require a thermal shock testing of -55℃ to 125℃ (so DT 180). The LED chip is sitting on a 2.3mm x 1.9mm AlN package (+/-3mm diagonal) . Now, we would like to know the thermal stress for this component on a FR4, Aluminum and Alumina PCB.
Using the formula above we are coming to following results:
|PCB material||Thermal stress (ppm*mm)|
It is clear that in this case the ceramic alumina PCB will give the best results of the three materials, reducing the thermal stress on the solder joint by 4 times over aluminum boards, hence reducing the risk for solder joint cracks significantly.
Thermal conductivity and CTE mismatch
The reason many components today are made out of Silicon, Aluminum Nitride or Gallium is not only for their low expansion material properties but also because of their thermal properties. Hence when selecting a board we also need to look at the thermal conductivity of the PCB material in combination with its CTE.
Whereas we see in this case that metals like aluminum and copper have a very high conductivity, we also notice their high CTE which makes them less suitable for applications with regular and high temperature variations. FR4 could address this better if it wasn’t not for its high thermal resistance, even though this can be improved by inserting thermal vias. The best option in this case is to go for a ceramic based board like Alumina (Al2O3) or Aluminium Nitride (AlN)
Fig 2: Thermal properties of existing manufactured thermal control materials, in relation to thermal properties of silicon and gallium arsenide. source (3)
As a specialist in different PCB substrate materials like FR4, ceramic and metal core (IMS) PCBs, Elite Advanced Technologies can help you to compare and decide the most suitable material. Contact us today for more information.
Sources and interesting further reading:
- (1) Tarr M., Failure in solder joints, can be found at http://www.mtarr.co.uk/courses/ami4812_map2/restricted/u08/fsj/index.asp
- (2) KENDZIORSKI (2011), Copper Leadframes Eclipse Alternatives for Military Memory Designs, can be found at http://www.cotsjournalonline.com/articles/view/101917
- (3) David L. Saums (2011), Technical Brief: Developments in CTE-Matched Thermal Core Printed Circuit Boards, can be found at http://www.electronics-cooling.com/2011/06/technical-brief-developments-in-cte-matched-thermal-core-printed-circuit-boards/
- Derek W Palmer (2011), Electronic Energy Levels in Group-III Nitrides, can found at http://www.semiconductors.co.uk/nitrides.htm#GaN
- Han B. (2003) THERMAL STRESSES IN MICROELECTRONICS SUBASSEMBLIES: QUANTITATIVE CHARACTERIZATION USING PHOTOMECHANICS METHODS, can be found at http://terpconnect.umd.edu/~bthan/paper/review/review.pdf
- Tarzwell B (2010), PCB 101: Coefficient of Thermal Expansion, can be found at http://pcb.iconnect007.media/index.php/article/54474/pcb-101-coefficient-of-thermal-expansion/54477/?skin=pcb
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