Posted on 16 Jun, 2016 by Russell Poppe
In a previous post, we discussed soak testing, a form of performance testing that is used in electronics manufacturing to find out how a product "survives" in the wild.
But what happens when the going gets really tough – how can you be sure that a product will be able to withstand more extreme scenarios? That’s where stress screening comes into play.
Unlike soak testing, which places a product under "normal" conditions, stress screening, also known as fatigue testing, imposes more punishing conditions that are designed to test a product to its limits - or beyond. For example, a product might be subject to vibration tests, thermal shock or mechanical shock.
Screen testing proves a product’s reliability, durability and safety by identifying the points at which failure will occur; the weak spots. So how is this process implemented by electronics manufacturers?
How stress screening works
There are two types of this form of testing:
1. HALT (highly accelerated life testing)
2. HASS (highly accelerated stress screening)
HALT is designed to literally test a product to its breaking point. In industries such as defence and aerospace engineering, where high reliability is absolutely crucial, it’s important to know exactly what conditions a product can stand. HALT allows you to identify the weakest points of a product and then work out how you can improve them. A product may go through several rounds of HALT, depending on how robust it needs to be.
HASS is comparable to an intensive soak test. For example, a product might be subject to the stresses endured during years of normal conditions in just a few minutes. However, a product should survive HASS testing without any damage.
Extreme testing in practice
Fatigue testing is highly bespoke. So if you are an original equipment manufacturer (OEM) outsourcing to an electronics manufacturing services (EMS) provider, it is important to specify your requirements. In what ways do you want to push your product to its limits? At what point is it acceptable for the product to "fail"? And at what point is it non-negotiable that the product "pass"?
One of the best examples of this is the extreme tests commercial aircraft are subjected to. Manufacturers need to be sure that aeroplanes can withstand turbulence, extreme weather, and forces during take-off and landing, among other variables.
This is how Airbus describes the process for its A380 model: "The A380’s fatigue testing lasted 26 months and was conducted to 2.5 times the design service goal. Testing accumulated a total of 47,500 flight cycles: 2.5 times the number of flights that an A380 would make in 25 years of operations. A 16-hour flight was simulated in just 11 minutes. The tests pushed the aircraft structure to its limits to identify any necessary design improvements. Final test and preparation for flight is a phase that includes calibration of the gauges, cabin pressurisation testing and testing of navigation systems."
As can be seen, the Airbus A380 was pushed beyond the limits it would be expected to withstand during its life. Both HALT and HASS are evident in the example above. These processes were achieved by placing a combination of loads on the airframe and activating computer-operated hydraulic jacks.
A smaller electronic product might be placed in a special chamber. For example, Cincinnati Sub-Zero (CSZ) sells HALT & HASS Time Compressor® chambers, which simultaneously subject products to "all-axis broadband vibration and rapid thermal cycling", in order to identify faults.
Overall, the most importance outcome of stress testing is the assurance that the manufactured product will function optimally when it reaches the end customer – even in extraordinary conditions.
Stress screening quite literally puts a product under stress, pushing it to its workable limits, to find out at what point it will falter. If a product buckles too early on in the process, changes will need to be made. Successful fatigue testing demonstrates that a product can cope with intense conditions if the situation arises.
If you outsource your manufacturing to an EMS partner it's recommended that you talk to them at the earliest opportunity, ideally at the product development stage, in order to develop test strategies on a product-by-product basis. You will no doubt find that they have a variety of test techniques and capabilities at their disposal – many of which are discussed in our eBook below, which we hope you will find helpful.