One of the reliability concerns of microelectronic circuitry is the effect of temperature change on components that may occur during manufacture, storage or actual use. Since all materials expand and contract at different rates, their different Co e f f i c i e n t s of Thermal E x p a n s i o n (CTE) lead to m e c h a n i c a l stress on the circuit board. This can result in failures due to solder cracking or lead fracture.

As seen in Table 1, the greatest difference in CTE of some typical circuit board materials is due to the low CTE of the silicon device. Thermal cycling is a test to determine the compatibility of electronic systems to the extremes of high and low temperatures relatively quickly. As components heat up and cool down they expand and contract causing fatigue or adhesion failure over time. Through repetitive cycling, the electronics are rapidly “aged” allowing the early discovery of material incompatibilities and/or potential failures in the field and the subsequent redesign of boards to provide high reliability.

Each temperature cycle consists of a low temperature soak, a transition to a high temperature, a high temperature soak, and then a return to the low temperature to repeat the cycle. This cycle is performed at temperatures and rates depending on the electronics under test and usually in accordance to a specified standard such as Mil-Std-883E 1010.8. EMPF has thermal chambers capable of a temperature range of -65 to 155ºC with a maximum ramp rate of approximately 10ºC/minute heating and 5ºC/minute cooling.

Thermal Shock is similar to Thermal Cycling but the transition time between temperature extremes is much shorter, as little as 5 seconds. This severe “shock” in temperature is achieved by rapidly moving the sample between two chambers filled with liquid or air held at the temperature extremes. The rapid temperature change can accelerate any inherent stress related problems on the board allowing early detection. EMPF can perform thermal shock in liquid from -75 to 160ºC per Mil-Std 883E 1011.9. Highly Accelerated Stress Testing(HAST) exposes circuitry to high temperature, high humidity, and high pressure simultaneously.

This evaluates the reliability of devices by accelerating the penetration of moisture through protective materials and activating any corrosion mechanisms due to moisture. The HAST chamber at ACI has a maximum temperature of 143ºC, a relative humidity level controlled between 75% and 98%, and a pressure in the range of 0.02 to 0.2 MPa. ACI performs testing to JESD22-A110-B.

Setup
Before testing, verification of chamber performance must be assured. Chamber size, sample size, and sample location within the chamber must all be considered during setup. Even with a circulation fan and microprocessor control, the temperature profile achieved is only highly accurate at the location of the manufacturer’s thermocouples. The actual hardware under test or a sample of similar size and thermal mass should be monitored with a thermocouple. For example, the analysis of our large thermal cycling chamber using a small test circuit board showed this temperature profile of -40 ºC to +85 ºC (Figure 1).The temperature in the chamber was controlled within a narrow range by cycling the heating and cooling equipment on and off. This produced the temperature oscillations at the sample thermocouple as shown in Figure 2. This is well within the typical 10 ºC spread allowed by the MIL-STD for thermal cycling.

Analysis
After the final cycle, the samples are typically examined under 10X to 20X magnification or higher to observe any mechanical failures. Figure 3 shows a crack at the solder joint in a Ball Grid Array solder ball after thermal cycling. The thermal cycles caused the solder joints of the BGA to expand and contract, continually stressing the joint until failure occurs seen typically as cracks in the solder connection. Electrical testing can also be performed to determine any performance change or electrical change in resistivity across the junction.

Conclusion
Thermal testing is a critical tool to certify the reliability of electronic circuitry. By carefully selecting thermal cycle parameters according to the potential use conditions, ACI can accelerate the testing and analysis of components to ensure a reliable product.