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Functional Test Target: MRB PCBAs

April 4, 2016

The Material Review Board (MRB) usually consists of a meeting of personnel from Quality, Materials, Purchasing and Manufacturing engineering. The purpose of an MRB is to review rejected material for disposition. Disposition decisions are often made based on schedule requirements, material cost, availability of alternatives, or analysis of the defect. The MRB material of interest is Printed Circuit Board Assemblies (PCBAs). The analysis of a defective PCBA often requires debug time from technical personnel, which is usually in very tight supply for volume manufacturers. This discussion focuses on utilizing functional test systems for MRB disposition in addition to development and process improvements to reduce PCBA functional failures.

The end goal of a manufacturing process is to manufacture defect-free product. Functional test is a necessity to achieving that goal by bringing PCBA defects to light via a quantifiable and repeatable process.

An overview of the ideal situation is; a functional test system at the PCBA manufacturing location and an identical functional test system available for program development and support for quality or test engineering personnel, both systems having identical fixtures and test software. A PCBA in MRB review could then be tested on the engineering test system for failure identification. If the PCBA fails on the engineering test system, the failure information can be sent to the PCBA manufacturing location with corrective action as the defect is repeatable on their test system.

The golden opportunity in targeting MRB PCBAs is that identified defective PCBAs have been culled by the manufacturing process and are available for examination. If we were hunting, the game just walked in front of us. If we were fishing, the fish just moved under our boat. Functional test provides quantifiable visibility to the defects of PCBAs like a scope for the hunter and a fish-finder for the fisherman.

Keep in mind that not all failures will be caused by PCBA manufacturing defects. PCBA failures in a manufacturing process can also be due to interoperability with other subassemblies or components, system level design deficiencies, tolerance build up or the manufacturing process itself. MRB PCBAs do contain many answers to questions on process improvement, if we can see what is really happening.

It is clear that functional test is an important part of PCBA manufacturing from a technical viewpoint, however companies make investment decisions based upon cost justification.  The following bullet point processes are intended to assist the reader to develop the potential impact of functional test from cost justification, product development and process control viewpoints.

Cost Justification:

How can the implementation of functional PCBA testing directly affect the profitability of the company?

  1. Find out the annual dollar value of scrapped PCBAs.
  2. Review MRB documentation to find the “heavy hitters” i.e. costing the company the most money to scrap.
  3. Investigate the stated cause for the PCBA failures among the “heavy hitters.”
  4. Determine if instrumentation could have been used to detect these defects at the subassembly level.
  5. How much money would the company save if these defects didn’t reach the final line?

Product Development:

How do I know that production PCBAs are performing the same as prototypes built in the lab?

  1. Functional PCBA test equipment measures and records critical voltages, currents, frequencies, pulse width, amplitude and period at test points throughout the PCBA (when using a bed-of-nails test fixture).
  2. Collect and examine data from volume production and compare it with tests run during development in the lab.
  3. Apply SPC principles to the collected data to identify variability in the initial (or subsequent) production runs.
  4. This process will “connect” the design effort to the shipping product, thus assuring the design is correctly implemented by manufacturing processes and volume component selections.

Process Control Improvement:

How do I know the PCBA process is in control?

  1. As production runs continue, compare the parameters of MRB PCBAs and production PCBAs with the original design data or initial production run data.
  2. Examine the data to identify changes, which may be due to process changes, component vendor change or tolerance build-up.
  3. Address these changes to improve yields, product performance and profitability.

The disposition of the rejected PCBAs in MRB is now either return to vendor, rework, scrap, use-as-is, or test it on a functional test system to see what caused this failure and FIX IT!

Related Links:

Electronics Functional Test

Economics of Electronic Subassembly Functional Test

Benefits of Universal Test for High-Mix Electronics Manufacturing

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