SMT- AOI System Alternatives
Quality control is key to effective competition in the international market. Specific tools such as statistical quality control (SQC), inspection, repair and test to control product quality are needed as well as having a quality system in place to ensure that products are built to meet the required quality levels on a consistent basis.
How should one accomplish this goal? First, SQC should be used to minimize the defects in assemblies. Either visual or automated inspection and test is used to ascertain the quality of products in accordance with established requirements. When those requirements are not met, repair is needed to ensure compliance. Defects that slip by during incoming and assembly inspections generally are found by electrical tests.
Quality control, inspection, test and repair issues are interrelated and must be treated as a system. If we are to manufacture reliable products at the lowest possible cost, we cannot view these areas in isolation. The resources spent in pursuit of goals in one dimension directly affect the others.
There are various inspection and test methods used in the industry. The visual inspection method is the most common and inexpensive but is very operator-dependent. The X-ray method is expensive, slow and has limited capability. Automated optical inspection (AOI) is faster but expensive. And in-circuit test (ICT) and functional tests sometimes also are used as inspection tools but offer limited capabilities. In my last column (SMT Magazine, March 2001, p. XX-XX), I discussed AOI systems. In this column, I want to look at X-ray and ICT as inspection methods. Note that AOI systems can be placed at different locations in the SMT line, while X-ray and ICT generally are used at the end of the line primarily as defect detection methods.
The most common and widely used inspection method is visual inspection. However, the main problem with visual inspection is that it is operator-dependent and, hence, subjective. For example, in one of the process control studies that I conducted1, the same operator performed all wavesoldering operations but at least 10 inspectors inspected the assemblies. When it appeared that these inspectors would report different quality levels, even if looking at the same assembly, we analyzed the data for three months and found that one of the inspectors reported three times more defects than the average of all inspectors. When this analysis was expanded to defect data for nine months, a wide variation in reported defects still existed. Further analysis of defect data was conducted by charting defects by each inspector for different part numbers of the same program. Again the same inspector consistently reported more defects than the others. Another inspector consistently reported fewer defects than the others, and the remaining inspectors showed wide fluctuations. A normal response to this situation is to reduce the human factor and switch to one of many automated inspection systems (discussed next).
Transmission Xray systems. These systems are more commonly used but have their problems as well. The images generated by transmission X-ray systems are due to the combined attenuation of the X-ray beam by every feature along its path. This type of system is good for detecting insufficient solder and voids in joints (as in detecting cavities in teeth) but cannot detect a nonwetted joint.
Also, when inspecting high-temperature and high-density noncollapsible solder balls (90Pb/10Sn) used in ceramic ball grid arrays (BGA), this system has difficulty detecting problems in the lower-density eutectic solder material (37Pb/63Sn). The high-lead content balls obscure the eutectic solder fillets. Transmission X-ray systems also cannot reliably assess joint quality on double-sided boards because intervening board and component material can diminish joint images significantly.
Scanned Beam Laminography (SBL). To overcome the problem of transmission microscopy, Hewlett-Packard makes what is commonly referred to as the SBL system. It also is referred to as the X-ray laminography system. SBL is a technique incorporating a microfocusing X-ray system to automatically generate and analyze cross-sectional solder joint images.
SBL creates horizontal cross-sectional images by scanning an X-ray beam about a vertical axis in sync with a rotating X-ray detector screen. By taking a cross-sectional image at different heights (pad, lead/ball and component), it can measure the amount and location of solder joints. Solder is imaged as darker pixels while less dense material (insufficient solder or voids) is lighter. The thickness of an unknown object is compared to the thickness of a set of known test samples.
The off-axis image formed on the detector is integrated through one or more rotations to generate a cross-sectional image of the horizontal plane in which the X-ray beam intersects the vertical axis. Additionally, SBL defocuses planes above and below a given plane to blur features outside the focal plane. This is very critical in isolating the plane of interest in solder joints from other material above and below it. For example, a laminographic image (cross-section taken at pad level) detects insufficient solder and vias disappear because they are not in the cross-section.
On the other hand, transmission X-ray images of the same board cannot detect the insufficient joint because the BGA ball hides it. This will be especially true if high temperature balls are used. However, transmission X-ray images can and do indicate insufficient solder if the solder balls reflow.
In most cases, to establish the actual size and shape of the hidden solder joint, transmission microscopy must be complemented with cross-sectional analysis ¾ a destructive test. In X-ray laminography, moving the board in small vertical increments through the focal plane allows different cross-sections of the same solder joint to be examined nondestructively. Large vertical movements of the board through the focal plane examine solder joints on both sides of a double-sided board.
Using such a system, users can establish tolerance bands for certain defects such as insufficient solder, and can identify marginal joints that should be further examined. The drawback is that it takes considerable time to develop the necessary accept/reject criteria. Additionally, if such a system were to be used for 100 percent in-line inspection, it could become a bottleneck in high-volume manufacturing.
However, many users have found the X-ray laminography system to be a good process control system when used to inspect boards on a sample basis or to inspect only a few of the critical components such as ultra-fine-pitch and BGAs on every board. When used in this manner, it can be a great tool for process development, characterization and monitoring to ensure that processes such as paste printing, placement and reflow are under control.
Be that as it may, transmission X-rays are used more widely than X-ray laminography. The main reasons are cost and the time necessary to program laminography systems.
Despite the availability of various inspection systems, many companies use automated test equipment (ATE) not only for testing but also as an alternate inspection system. Using ATE equipment for inspection is limited in that the system only can be used to find opens and shorts, as far as manufacturing defects are concerned. So there is some push by inspection system manufacturers to replace the ATE equipment with their inspection systems. In reality, the functions of each of these equipment types are different; they complement each other.
Functional tests in which assemblies are run through the connector provide moderate fault coverage, which can be improved if the board is designed for system-level test. Generally, the diagnostic accuracy of functional or system-level tests is limited to functional blocks on the board instead of component or solder joint level (i.e., manufacturing) defects. Because the process newness increases the potential for this type of defect, functional tests are not considered effective for SMT boards.
Solder joint inspection is an after-the-fact step. A more effective practice is to take preventive action ¾ that is, implement process control to ensure that problems do not occur. Does this mean that inspection is not necessary? Far from it. Inspection must continue completing the loop on defect collection, monitoring the process and implementing corrective action so that problems do not recur.
1Ray P. Prasad, Surface Mount Technology: Principles and Practice, Chapter 14, Inspection, Test and Repair.
RAY P. PRASAD is an SMT Editorial Advisory Board member and author of the textbook Surface Mount Technology: Principles and Practice. Additionally, he is president of BeamWorks Inc. (www.beamworks.com), a supplier of selective automated assembly systems, located in Portland, OR and founder of the Ray Prasad Consultancy Group, which specializes in helping companies establish strong internal SMT infrastructure. Contact him at P.O. Box 219179, Portland, OR 97225; (503) 297-5898 or (503) 646-3224; Fax (503) 297-0330; Web site: www.rayprasad.com.
AOI systems can be placed at different locations in the SMT line.