Reading time ( words)
By Ray Prasad
Specific tools such as statistical quality control (SQC), inspection, repair and test are not only needed, they should be in place to ensure that products are built to meet required quality levels on a consistent basis. How should this goal be accomplished? First, SQC should be used to minimize the defects in assemblies. Visual or automated inspection and test can ascertain the quality of products. Defects that slip by during inspection are generally found by electrical test.
Quality control, inspection, test and repair are interrelated and must be treated as a system. If reliable products are to be manufactured at the lowest possible cost, these areas cannot be viewed in isolation. Resources spent in pursuit of goals in one dimension directly affect the others. This article will focus only on inspection and test issues.
Visual inspection is the most common and inexpensive inspection method; it is operator dependent. X-ray is expensive, slow and has limited capability. Automated optical inspection (AOI) is faster but expensive. In-circuit test (ICT) is also used as an inspection tool.
Figure 1. Visual inspection equipment.
Visual InspectionThe most widely used method of inspection is visual inspection with a magnifier or microscope at 2 to 10X magnification (Figure 1). J-STD-001 requires inspection at 2 to 4X for all devices with lead pitches greater than 0.020". For fine-pitch devices with lead pitches of 0.020" and below, 10X magnification is required. Higher magnification should be used for reference only.The main problem with visual inspection is that it is operator dependent and subjective. For example, if the same assembly is given to different inspectors, they will report different quality levels. An analysis of defect data was conducted by charting defects by each inspector for different part numbers of the same program. The results of four inspectors are plotted in Figure 2.1 Inspector D consistently reported more defects than the others did. Inspector E consistently reported fewer defects than the other inspectors, and inspectors C and G showed wide fluctuations. Switching to an automated inspection system would reduce the subjective nature of visual inspection.
Automated InspectionBecause the industry has been moving toward finer pitches and ball grid arrays (BGA), visual inspection of solder joints is either very difficult or impossible. Even when feasible, it is too risky because of the human factor.
AOI systems utilize image sensor technology with computer analysis of the digital images. Today there are numerous systems on the market with a wide range in price and accessories.
Figure 2. Solder defects vs. operators on identical assemblies.1
When selecting the machine, decide what the AOI system should do. For example, should the machine point out missing components, component polarity, placement accuracy, paste deposit or solder joint quality? Keep in mind that most machines do a decent job when it comes to identifying wrong polarity or missing components, but they may be challenged to accurately identify solder joint quality.
No matter what type of equipment is used, the general requirements of AOI systems should be based on accuracy, repeatability, speed, computer-aided design (CAD) compatibility, and false fails and false accepts.
It is sometimes assumed that AOI systems can be used for process control by changing the appropriate variables to correct defects on a real-time basis. This is wishful thinking many of the changes necessary to prevent the problems require human intervention and engineering judgment. There is no inspection system that can pinpoint a given defect and identify its cause alone. For example, AOI systems largely depend upon mass and solder density (insufficient, excess or none). Defects having more than one cause could never be placed into only one of these three categories and meet the needs of an AOI system. For example, voids, insufficient fillet and insufficient solder paste all cause insufficient solder. The corrective actions for these defects are different, calling for human judgment and intervention.
Inspection Philosophy: Prevention or Detection?Current inspection philosophies believe that it can either be used for prevention or detection of defects. Prevention of defects is the more appropriate approach because the focus is on process control and elimination of defects by implementing corrective action. In such an approach, AOI machines are placed right after the solder paste printer and after component placement.
When inspection is used to detect defects, it is the last step in the manufacturing process, prohibiting bad product from leaving the factory. The inspection machines are placed after the reflow oven.
Assembly TestingDespite the availability of various types of inspection systems, many companies use automated test equipment (ATE) not only for testing, but also as an alternative to an inspection system. The main limitation of using ATE for inspection is that the equipment is used primarily to find opens and shorts. There has been a push by some inspection system manufacturers to replace ATE with inspection systems. In reality, the function of each piece of equipment is different; they complement each other.
Design for testability (DFT) is more important than ever for SMT. For this reason, the test engineer, as well as the manufacturing engineer, must be integral parts of the product team. Requirements for manufacturability and testing should be considered at the design stage. Assume that the product meets the design guidelines.
Functional tests, in which assemblies are run through the connector, provide moderate fault coverage that can be improved if the board is designed for system-level test. Generally, the diagnostic accuracy of functional or system-level test is limited to functional blocks on the board instead of component- or solder-joint-level defects. Because the newness of the process increases the potential for this type of defect, functional tests are not considered to be very effective for SMT boards.
The defects encountered in SMT solder opens, solder wicking, tombstoning, bridges, misalignment, part movement, solder balls, etc. are caused by solderability and lead coplanarity problems, poor paste printing, bad placement and improper soldering process profiles. These phenomena translate into two major defect types: opens and shorts.
ATE, also known as ICT, bed-of-nails testing and pogo-pin testing, is the best way to detect the manufacturing defects of opens and shorts. To ensure that ATE can test the board, access to test nodes must be provided in the design.
In ATE, the test probes are sharp and can easily damage the surface if improperly applied. They must not touch the brittle top passivation layer of the resistors; not only might the surface be damaged, but if some of the resistive material is chipped away, the resistive value might change. Also, to guard against false-positive results, the test probes must not touch the leads or terminations of the components. If the test probe is pressed against an improperly aligned part, the probe itself may be bent or damaged.
Issues in ATE TestingWith increased functional density and reduced interpackage spacing, the use of 0.050" probes is essential. Overall, small probes tend to increase the cost of the test fixture. In addition, there is the question of accuracy in the test fixtures. Smaller probes allow smaller test pads (0.030 to 0.032" with 0.018" holes). Some companies have used 0.025" pads with 0.012" holes. It is difficult to accurately align test probes on the smallest test pads; larger pads reduce the probability of missed pads.
The alignment problem on smaller test pads is compounded because, unlike board fabrication and assembly vendors, test-fixture vendors do not follow a test-fixture specification. Very few companies have an in-house test-fixture specification and industry standards are lacking.
The accuracy problem is further compounded by the method of inserting receptacles. Hardly anyone auto-inserts receptacles that are as large as 0.100". The accurate insertion of 0.050" receptacles is a matter of the right "touch," or combination of experience and care.
Because almost all vendors use the same method of receptacle insertion, the alternative of finding another vendor does not exist. One must work very closely with the selected vendor to develop a workable fixture specification to meet the requirements.
When users and vendors have agreed on a fixture specification, they must establish a method for determining the accuracy of and compliance to the specification. Some methods for measuring accuracy include video-equipped coordinate measurement, milling machines and transparent alignment plates.
ConclusionSolder joint inspection is an after-the-fact step. A more effective practice is to take preventive action. Does this mean that inspection is not necessary? Far from it. Inspection will continue to complete the loop on defect collection, serve as a process monitor and be used to implement corrective action.
REFERENCE1 Ray P. Prasad, "Quality Control, Inspection, Repair and Test," Chapter 14, Surface Mount Technology: Principles and Practice, 2nd edition, Walter Kluwer Academic Publishers, ISBN: 0-412-12921-3.
RAY P. PRASAD is an SMT Editorial Advisory Board member and author of the text book Surface Mount Technology: Principles and Practice. Contact him at P.O. Box 219179, Portland, OR 97225; (503) 297-5898; Fax: (503) 297-0330; Web site: www.rayprasad.com.