Back end testing for packaged devices

Power MOSFETs need a lot of different tests. We understand backend test as we have over 80% of our installations used in this application. Our customers operate 24hrs every day delivering fully tested and QA approved products at throughput rates up to 20kuph. Backend test, for power devices in particular, has a diverse but well-established list of technical demands to detect failures and failure mechanisms.

Our Mostrak-2 (M2) system, provides all the tests for power MOSFETs, in parallel at the fastest possible throughput rates, using test type-specific resources. These test groups may be summarised as follows:

Static Parameter tests
All discrete devices require verification that they meet the parametric specification, by eliminating any that may have been damaged by the assembly process.

Transient Thermal tests
All power discrete devices need verification that the bonding processes have been successful during assembly.

Avalanche Energy tests
MOSFETs especially, need a functional test to verify the ruggedness necessary for inductive load applications. Our UIS generator handles this testing.

Dynamic Switching tests
Some specific applications require that dynamic characteristics are measured to verify switching energy loss levels. Our DS5 test generator is capable of performing this testing.

Enabling technology — ip Mostrak Architecture

Combining all these test groups in one test system, at economic throughput rates is a challenge but that is exactly why the ip architecture was developed.

The ipTEST indexed parallel architecture was developed specifically to provide the fastest possible solution for packaged device testing. The idea of partitioning the test programme, and performing the tests on multiple in-line test heads was not new when ip architecture was conceived. It was however the first, and remains the only single platform, fully integral tester, with all the test functions under one central controller, and just one test programme. This is a true "one-stop shop" tester.

Mostrak for backend can be specified with up to 8 test resources that can be used on 1 to 4 handlers.

In-line QA tests and data

Usually this is a second set of static parameter tests to fulfil outgoing quality standards. Our QASort software allows for the integral or separate control of QA tests and interpretation of the data. There is also an AutoSample facility to provide a statistical third test check in-line.

Throughput is higher

When interfaced to the most productive handlers Mostrak can almost always drive them at full speed. The next generation Mostrak-2 (M2) system is our fastest production speed tester. We are achieving the following:

*When power tests are not required, Mostrak can accommodate more static test resources and drive additional handlers. Multiple handlers may be interfaced for true independent parallel operation, as follows:

2 handlers with both Production and QA static tests
4 handlers with Production static tests only

Productivity therefore on low power devices per test system can be as follows:

Automation

Over the years we have developed an understanding of the complete backend process, and our aim is to ensure that the test system becomes a seamless part of the production process and uses the latest advances in automated programming.

Productive up-time is maximised

For use with intelligent handlers, we have developed a Test Cell Controller software module that minimises operator intervention and error rate, and eliminates most of the dead time between lot changeovers. Cell controller software does the following:

• Bar code auto-load and auto-close for lot changeover, test programme etc.

• Auto download of laser mark codes

• On-line control of handler, mark and lead verification

• Auto purge at interruption or end of lot

• Custom screens and data transfer

Cell control can reduce lot changeover times from a typical 30 minutes to just 3 minutes.

Our system can be configured to drive virtually any handler platform economically. Most installations today are made with rotary turret handlers where high productivity can be combined with a small footprint at backend finishing, but there are many successful applications with other handler platforms.

Parallel Test with Strip or Carrier handlers

Mostrak is inherently parallel in operation. The system software allows for multiple tests in parallel and of course with appropriate hardware resources fitted, this makes strip or carrier test available now.

For example, a system equipped with 4 sets of static resources can test a column of 4 devices that have been electrically isolated at lead frame level, normally into a carrier. The carrier indexes column by column, thus achieving throughput rates that are not possible with single device handlers. Consider a Dpak lead frame with 4 x 25 devices handled as a single unit. Throughput rates in excess of 50,000uph are possible. No custom engineering is necessary; it's all ready today.

More than MOSFETs

Mostrak is a complete tester for all discrete components, including diodes, zeners, SCR's, triacs, bipolar transistors, and IGBTs. But it doesn't stop there. We recognise that where production line investment is in a particular package style, it is economically desirable to test all products in that particular package.

Our response has been to specially develop test resources for products that may need to share the backend process because of packaging. Two examples are Linear Voltage Regulators, and Transient Voltage Suppressors, where test capability is added for 3 to 5 pin variants of the various discrete packages.