Cutting the cost of Solder Joint Inspection

Article

Mayben, James E.   (1988, ASQC)   General Dynamics, Forth Worth Division, Forth Worth, TX

Annual Quality Congress, Dallas TX    Vol. 42    No. 0
QICID: 3474    May 1988    pp. 535-540
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Article Abstract

This paper deals with the feasibility of using laser-based automatic equipment to inspect plated-through hole solder joints in printed circuit board assemblies for production acceptance. A laser heats the solder joints for a short period of time. The cooling rates of the solder joints are then monitored in terms of infra red emission by these joints and measured in terms of Peak Thermal Units (P-units).

Based on peak thermal units, solder joints which exhibit abnormal cooling rates are considered marginal joints as opposed to the ones with average cooling rates which are considered acceptable. A computer is used to collect data, monitor precise heating and segregate solder joints with abnormal cooling rates from the ones with average cooling rates. Recursive statistics have been applied to the cooling rates to provide the statistical significance for definition of acceptable solder joints as compared to margin solder joints.

The Laser Inspection system can inspect plated-through holes of a printed circuit board (PCB) assembly with consistently higher accuracy and at a higher rate than visual inspection. The automation of the inspection of solder joints of PCB assemblies was desirable to:

  • a. Increase throughout
  • b. Provide consistency in inspection of solder joints
  • c. Increase quality and reliability of solder joints
  • d. Reduce costs
The Laser Inspection system exhibits such capabilities. Consequently, an in-depth study was undertaken to determine its feasibility for use with the General Dynamics Fort Worth Division electronics manufacturing process.

This study entailed the usage of the Laser Inspection system to inspect solder joints of plated-through holes of PCB assemblies in the production environment of the General Dynamics Fort Worth Division Electronics Manufacturing Center. The study was divided into three major sections:

  • a. Technology application study
  • b. System's interface with the manufacturing process
  • c. Development of accept/reject criteria
A summary of the essential results arrived at were as follows:
  • a. A Non-Destructive Test specification for accept/reject criteria was generated. This specification was applied to existing PCB assemblies by the release of an engineering change which specifies Laser Inspection as an acceptable alternate to visual inspection.
  • b. Variability within the soldering process was optimized for a high acceptance rate using the Laser Inspection system as a process control tool.
  • c. The Laser Inspection system removes subjectivity that is present with visual inspection of solder joints and replaces it with a repeatable inspection technique.
  • d. Quality and reliability of laser inspected/accepted solder joints is improved.
This paper further discusses limitations of the Laser Inspection system and outlines those conventional inspection/test methods which must be retained in the process. It also summarizes the advantages/disadvantages of the Laser Inspection system including the cost benefits and throughput increases.

Keywords

Aerospace industry,Department of Defense (DOD),Inspection,Printed circuit board (PCB)


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