HAST is the highly accelerated stress test, a method that accelerates ageing of a product by applying high temperature, high humidity and pressure. It evaluates the reliability of non-hermetic packaged devices in humid environments. Severe temperature, humidity and bias conditions accelerate the penetration of moisture through the external protective material or along the interface between the protective material and the metal conductors. UHAST applies no bias voltage, ensuring that failure mechanisms which could be masked by bias, such as galvanic corrosion, are detected.

1. Advantages of HAST

(1) Higher product reliability. Because HAST uses a step stress approach, continuously raising stress to reveal and correct the defects it excites, product design becomes increasingly complete, pushing both the operating stress limit and the destruct stress limit far above the stresses specified in standards or encountered in use and producing a very large design margin. Highly accelerated stress screening then removes potential defects introduced in manufacturing, further improving reliability.

(2) Shorter development time and lower cost. Accelerated testing brings a product to design and process maturity far faster than conventional design and testing, shortening the overall development cycle so the product can reach the market sooner and capture share. Faster maturity of design and process actually lowers development and production cost.

(3) Simplified verification. Because HAST has already applied stresses well above all the maximum stresses specified in the standard, and because in general no further defects are found at lower stress levels, design verification or design qualification testing at the specified stress levels may be reduced or omitted.

2. Applications

HAST is widely used for accelerated ageing and life testing of products in industries including PCB, IC semiconductors, connectors, circuit boards, magnetic materials, polymer materials, EVA and photovoltaic modules, and to evaluate sealing, moisture absorption and ageing performance.

3. Purpose of HAST

HAST is a basic process for strengthening a design or eliminating early failures. Besides applying stress it involves five aspects: precipitating defects, detecting faults, failure analysis, taking corrective action and verifying corrective action.

(1) Precipitating defects: turning potential or undetectable defects into obvious or detectable ones by applying stress.

(2) Detecting faults: a potential defect becoming obvious does not mean it can actually be detected, so stress excitation and similar methods are used to put the obvious defect in the state where it is easiest to detect.

(3) Failure analysis: once a fault is detected, it must be analysed to identify the cause so that the appropriate improvement can be selected.

(4) Corrective action: improving the design or process so that the fault will not recur. Taking corrective action is the main purpose of HAST.

(5) Verifying corrective action: confirming by test whether the corrective action is effective.

4. The HAST process

In the R&D and trial production stage, HAST applies high temperature, high humidity and high pressure to accelerate moisture ingress and produce ion migration, quickly revealing and locating design and process defects, giving a strong basis for improving design and shortening test cycles. This brings design and manufacturing processes to maturity before mass production, so that design and process related failures rarely occur in the field and product competitiveness improves. For products already in service, HAST quickly reproduces field failures, shortening the improvement cycle and reducing after-sales cost.

5. Applicable standards

IEC 60749-4, highly accelerated stress test.

ED-4701/100A, unsaturated vapour pressure test.

JESD22-A118, unbiased highly accelerated stress test.

JESD22-A110E, highly accelerated stress test.

JESD22-A102E, unbiased autoclave test.

AEC-Q100 Revision H, biased and unbiased highly accelerated stress test.

JPCA-ET08, unsaturated pressurised steam test.