While burn-in has long been effective for improving product reliability, burn-in of power supplies creates an additional problem. It generates an enormous amount of heat that must be removed.
To address this problem, Ericsson Components drew on Schaffner’s experience in power supply test to implement a reliability stress screening (RSS) solution that offers major gains in operating efficiency. At the heart of this solution are new intelligent electronic loads that recycle power to reduce heat output and cut overall costs by as much as 85%. Management software that simplifies testing of large numbers of units and facilitates process monitoring completes the package.
RSS offers a crucial opportunity for raising product quality. It accelerates burn-in by as much as 66%, resulting in a more robust product; and increases field reliability by 200% to 500%, cutting factory rework and warranty costs dramatically. As a result, Ericsson replaced conventional burn-in with RSS several years ago.
Reliability Stress Screening
Ericsson’s RSS program for power supplies, power rectifiers and DC/DC
converters combines temperature stress and power switching with load cycling (Figure 1). More aggressive techniques, such as vibration and very high temperature stress, are the province of the design engineer.
The duration, number and sequence of temperature, power and load cycles are functions of the target environment of the power supply. For Ericsson, this target environment can vary from the inside of a mobile phone to a radio base station at a remote location or in the relatively benign surroundings of a central office switch.
Power cycling at predetermined points in the RSS cycle creates high thermal gradients in weak solder joints and poor bonds. This is particularly effective in power supplies where surface-mount technology is used extensively, because simple heat-soak techniques do not create sufficient stress.
Thermal cycling is highly successful for screening small open assemblies, the essence of many power supplies and DC/DC converters. The rapid change in ambient temperature around the unit creates thermal displacement at the junctions of dissimilar materials. The faster the rate of change, the greater the stress, flushing out intermittent failures and unacceptable component value drift.
Load cycling—ramping a load connected across the unit-under test’s (UUT) output terminals up and down in synchronization with power and temperature cycling—highlights marginal components quickly. These loads are normally resistive in nature, and their only available means of dissipating the output power from the UUT is to convert it into heat. During stress screening when many units are tested in parallel, this heat must be removed.
To alleviate this problem, Schaffner introduced a regenerative load that, instead of converting power into heat, recycles up to 85% of the power back into the mains (Figure 2). As part of its RSS program, Ericsson installed 54 regenerative loads at one of its largest power-system manufacturing facilities.
Recycled Power
The key to the regenerative load is a power-recycling unit supporting four programmable 5-kW electronic loads. When one of these loads is connected across the output of a power rectifier, the output of the load is routed into the recycling unit and returned to the mains (Figure 3).
Under program control by the PC, the load operates as a controlled current sink, passing the output power to the DC/AC regeneration unit. The regeneration unit is synchronized with the AC mains. A 12-pulse thyristor design is used for high power loads, while lower power loads use a high-frequency pulse-width-modulation scheme to provide 3-phase power-factor correction as well as high dynamic response.
The combined efficiencies of the electronic load and the recycling unit recycled at least 85% of the current. This reduced the power consumption and dissipation of the 20-kW load to less than 3 kW. To further minimize power losses due to long cables, Ericsson mounted all the regenerative loads immediately above the respective climate chambers.
With its highly varying product mix—for example, one Ericsson facility manufactures rectifiers and DC/DC converters ranging from 50 W to 7 kW—the company expects to achieve average chamber utilization of 50% to 80%. Assuming 24-hour burn-in for 300 days per year at 75% utilization and Swedish electricity costs of 5.4 ¢/kWh, each load could save around $5,000 per year.
Taking into account the savings on ventilation and depreciation of the new loads, the company expects the regenerative loads to cut RSS power consumption by 0.5 MW. This could save more than $250,000 annually when all systems are installed and running.
Ericsson’s RSS systems, comprised of a chamber containing three cabinets each fitted with three regenerative loads, are controlled by dedicated PCs that manage the RSS cycle and monitor the UUTs. The PCs are networked to a remote server that manages the RSS systems, supports test program generation and provides central storage and analysis of test data.
Monitoring and Managing
As each power supply or rectifier type has a different test time, the control software allows UUTs to be monitored and removed from the stress screening process without affecting others whose cycles have not yet finished. This enables Ericsson to maximize chamber utilization.
Schaffner’s Powerscreen software controls cycle start-up, UUT monitoring and removal, and higher-level functions such as chamber temperature and power on/off cycling. To simplify operator interaction, a radio-controlled bar-code reader inputs cabinet locations, identifies additions to and removals from cabinets, and issues commands such as Start and Verify. As each chamber starts, the required RSS programs are downloaded from the central server.
The server allows operators to interrogate the status of any UUT in any chamber to check voltage, current, temperature and pass/fail. This continuous status monitoring by the server also ensures that operator assistance can be requested if there is a fire, smoke or other user-defined alarm condition.
All RSS data is stored in a special data base, fully compatible with Ericsson’s overall factory data-management system. This allows the results from the RSS process to be integrated into the company’s quality-management program.
Dataview, Schaffner’s on-line monitoring package, allows supervisors to examine the contents of the data base without leaving their desks. Data is monitored by setting up a view. The user defines the view’s visual properties such as lists, graphs and displays; selects the data-base files and fields to be monitored, and activates the view. Views are refreshed or updated regularly.
In some cases, it may be more productive to monitor the state of a parameter rather than its value by defining a data query which triggers a warning if a particular condition has been reached. Each user-defined real-time view is effectively a separate window, and calling them up individually can be quite tedious.
To relieve this problem, several views can be combined in a workspace. Later, the workspace can be selected and the views loaded automatically.
The world’s increasing reliance on electronics is driving the industry to strive for ever higher quality. RSS offers a crucial opportunity for raising product quality and increasing field life. Schaffner’s regenerative loads and management software enable Ericsson to imple
March 1997