Burn-in and system-level test are the two insertions most often added by habit and most often resented on the cost line.
Both catch real failure modes. Neither is universally necessary. The decision should follow from what you are actually trying to screen out.
What Burn-In Screens
Burn-in targets infant mortality: the early-life portion of the failure curve where weak parts fail quickly rather than surviving. Parts are run biased at elevated temperature and voltage for hours to tens of hours, then re-tested.
The logic only holds if your population actually has a meaningful early-failure rate. On a mature process with good yield and no known defect mechanism, burn-in removes very little and costs on every unit.
What System-Level Test Catches
SLT runs the part in something resembling its end application — on a board, with firmware, sometimes with real workloads — rather than on a vector-driven ATE.
It catches what structured test misses: firmware and sequencing interactions, marginal timing under real traffic, power-management corner cases, and defects that only appear in a specific operating mode.
- Interaction defects between hardware, firmware and power management
- Marginal behaviour that pattern-based test does not excite
- Board-level and interface issues that ATE cannot reproduce
The Cost Is Real
Burn-in adds hours of oven time per unit plus extra handling and extra insertions. SLT adds board fixtures, firmware maintenance and test time that is usually longer than ATE time per part.
For high-volume consumer parts, even a small per-unit cost is a large annual number. For low-volume, high-value parts, the same cost is negligible against the price of one field failure.
A Decision Rule That Works
Use burn-in when you have evidence of early-life failures or a customer requirement that mandates it. Drop it when yield is stable, the process is mature, and the failure data does not show an infant-mortality signature.
Use SLT when the device is complex enough that pattern coverage alone leaves risk, or when field returns have historically been caused by interaction issues rather than structural defects.
