What is Proof of Screen?
Proof of Screen (POS) is the demonstration that a HASS profile is both effective and safe: it precipitates defects in production units without removing meaningful life from good product. It is done before the screen touches saleable product, and repeated when the screen or the design changes materially.
· Updated
Effectiveness: the screen finds defects
A screen that does not precipitate defects is only cost. Effectiveness is demonstrated by screening units with known or seeded defects, such as deliberately marginal joints or loosened hardware, or field returns with no-fault-found histories, or by confirming that the screen catches the defect types in the current escape history.
Safety: the screen does not use up good product
A screen aggressive enough to precipitate defects is, by definition, aggressive. Safety is demonstrated by running a small set of known-good units through many consecutive screens, 20 to 50 repetitions being common practice, with full functional verification. Survival with margin shows that a single screen consumes only a small fraction of product life. Where practical, a HALT on screened and unscreened units then confirms that the limits have not degraded.
Why it cannot be skipped
Guard-banding the HALT limits by some arbitrary percentage is not the same thing as Proof of Screen, and is not a substitute for it. Without it, you do not know whether you are screening or slowly damaging inventory. Proof of Screen also presupposes a HALT-matured design: a HASS screen lives in the operating margin, above specification and below the operating limits, so a design with thin margins cannot be screened safely.
Document the results as the baseline authorization for the production screen. The step-by-step procedure, from setting initial screen levels to running HASS in production, is in the methodology guide.
In the standards
GM’s GMW8287, a guideline for HALT and HASS/HASA, includes this step as “Safety of HASS”: a full fixture of production parts runs the screen 20 times, with the aim that every part completes all 20 without incident. IEST-RP-PR003, IEST’s recommended practice for HALT and HASS, also covers HASS. The standards guide summarizes both.
The short answer
What is Proof of Screen?
The demonstration that a HASS profile finds defects (effectiveness) and does not consume significant product life (safety). Known-good units run through the screen many times, commonly 20 to 50, and units with known or seeded defects show that it catches them.