An overview of burn-in
To be commercially successful, optoelectronic devices need to be reliable. However devices fail due to defects which arise during the manufacturing process. Burn-in involves running components under a continuous period of operation under controlled conditions to identify infant mortalities (devices that fail very early in their use), and to stabilize operating parameters thus ensuring that every device shipped is up to standard. Manufacturers/suppliers of laser diodes must consistently provide quantitative assurances (test data) to prove the reliability of their devices. Having this reliability data is essential to device makers in winning repeat orders and can justify a premium on price.
Burn-in is frequently conducted under ‘stressed’ conditions, at an elevated temperature and perhaps elevated current. This process may also be called heat soaking. The components may be under continuous test, tested at the end of the burn-in period, or tested both at the beginning and end of the burn-in so that the change in operating parameters can be measured.
In theory, if the parameters of the burn-in are appropriate, virtually all the weak components will fail during the "Burn In" period allowing those parts to be discarded. Removing the weak components ensures the ones that pass are highly likely to operate successfully for their design lifetime with minimal failure in the field. A typical burn-in period would range between 12-24 hours.
The two most common types of burn-in are;
1. In Situ Burn-in
2. Burn-in with separate test (either just one at the end, or tests at the beginning as well as the end)
In Situ Burn-in
This is typically used in low volume production environments. Parametric tests are performed in the same system that is used for burn-in. Sometimes this is the equipment that the device is going to be in the field.
Burn-in with separate test
This is a more cost effective method used in high volume production environments. Parametric tests are performed in a different system than is used for burn-in.
The purpose of burn-in
One could say that the purpose of burning-in a laser comes down to two main reasons.
1. One is to identify devices with 'infant mortalities'
2. The other is to stabilize the operating parameters. For example, A a new laser might change its light output (assuming a constant current and temperature of course), over the first 100 hours by a rapid drop in power or in some cases, a significant increase in power. After this period, they become stable for a long time - sometimes in excess of 25 years, depending on the operating conditions. Higher currents and higher temperatures inevitably lead to shorter lifetimes.
Very commonly, a laser will include a monitor that measures the laser optical power with a photodiode that is incorporated into the laser package. Basic laser diode operating characteristics are measured by increasing forward current (I) while measuring the device voltage (V), light output (L), and monitor photodiode current (lpd). The resulting measurement is known as an LIV curve (fig 1).
(Figure 1: LIV Measurement performed on Yelo’s software)