Your incoming inspection passed it. Your AOI passed it. Your functional test passed it. Your burn-in passed it. It shipped. And three months later, a customer in Munich is on the phone because it failed — intermittent, temperature-dependent, and impossible to reproduce in the lab. That failure was always there. It was a latent defect. HASS — Highly Accelerated Stress Screening — is the production test designed to find those defects before they ship.
The relationship between HALT and HASS
HASS cannot exist without HALT. HALT testing characterises a product's fundamental stress limits during development — the temperatures and vibration levels at which it begins to malfunction and eventually fails permanently. Those limits define the Operating Limit and the Destruct Limit. HASS uses those limits to construct a production screen. The HASS profile applies stresses between the operating and destruct limits — severe enough to precipitate latent defects, but not severe enough to damage a good unit. That corridor is called the Screen Strength.
What the HASS screen looks like
A typical HASS profile runs for 30 to 90 minutes per unit. It combines thermal cycling at ramp rates of 40–60°C/min — far faster than conventional burn-in — with six-degree-of-freedom vibration at 10–20 Grms. Both stresses are applied simultaneously, with the product powered and under continuous functional monitoring. A joint that fails during a thermal ramp and recovers when temperature stabilises will show up as an intermittent failure. After-test-only electrical testing misses every intermittent defect.
HASS vs. burn-in
Most latent defects are mechanical — marginal solder joints, incompletely seated connectors, cold welds at wire terminations. Thermal stress alone does not efficiently stress mechanical interfaces. What does stress them is thermal cycling and vibration. A conventional burn-in oven applies neither. Studies comparing traditional burn-in to HASS have found that HASS screens at equivalent defect escape rates require roughly one-tenth the throughput time of equivalent burn-in programmes.
The screen survival test
Before deploying a HASS screen in production, a sample of known-good units is run through the proposed screen profile multiple times. Industry practice recommends 10 to 20 consecutive screen cycles. If the known-good units show no degradation after repeated screening, the screen profile is safe. If they show degradation, the screen is too aggressive and is consuming product life with every cycle — the exact opposite of what HASS is supposed to achieve.
What HASS finds that other screens don't
Marginal solder joints that passed AOI and X-ray but have insufficient mechanical integrity. Cold welds at wire terminations. Incompletely seated connectors that pass functional test but fail under multi-axis vibration. Component parametric shifts — capacitors with marginal temperature coefficients, oscillators that drift at temperature extremes. Continuous electrical monitoring during HASS catches these as functional failures during thermal excursions.
The programme that HASS doesn't replace
HASS screens production units. It does not replace the development reliability programme. A product with a fundamental design weakness will fail HASS screens consistently — the right response is to fix the design, not soften the screen. HALT finds design deficiencies. HASS finds manufacturing defects. Both are necessary. Neither replaces the other.