Visual stimuli dominate most people’s modes of information acquisition, allowing the absorption and filtration of pertinent data from our environment in a highly rapid manner. So much of our communication is tied up with our sight that we have developed languages and methods to transfer nonverbal ideas. Although this overreliance on visuals can be nerve-wracking during a first date or interview, it has proven itself as the time-tested process by which we begin to perceive the world around us.
Human vision is also necessary for assessing printed circuit boards during quality control. Though it may seem rudimentary, PCB visual inspection via manual visual inspection (or MVI) is the backbone of modern inspection techniques.
Wait, Isn’t There a Machine That Could Do This?
Despite widespread advancements in automation throughout the PCB manufacturing process, the industry still relies on manual visual inspection as its primary evaluation method. Process engineers have yet to develop a better visual analysis than the one used to read this page. However, it remains an area of heavy research, and progress is being made.
While it may be taken for granted at times, human visual acquisition and analysis remain extremely powerful techniques in the face of cutting-edge technology. With only a bit of training and magnification tools, human operators can detect defects at a high rate of efficiency and can better discern boundary cases of quality and different potential defect modes.
However, the drawback of biological visual systems is they are susceptible to entirely different vulnerabilities in defect detection than purely electromechanical ones. Thus requiring system designers to understand potential pitfalls and circumvent them:
- Ergonomic test equipment/environments: Human operators need sufficient support to maintain their accuracy and visual endurance for extended evaluation periods. Ironically, one of the most necessary tools can be among the most detrimental if not carefully calibrated: lighting. Light sources should be flexible to allow illumination from various positions and angles. Another significant concern is the quality and intensity of the light cast, and sources must balance glare against a minimum illumination level to prevent eye strain. Dimmers help testers achieve a comfortable level of intensity which can be quickly modulated as test conditions dictate.
- Magnification: Roughly, magnification tools can be sorted between high-powered microscopes and illuminated magnifiers that lessen exertion on the eyes during an inspection at the cost of a reduced resolution. Microscopes can operate fundamentally as binoculars for 2D image magnification or utilize two separate binoculars (one for each eye) that the brain processes as a 3D image for extra acuity. Test engineers may also want to consider a three-” eyepiece” microscope which can capture video for ease of dissemination.
More intense methods of imaging should be utilized for the defects that are undetectable either with standard levels of magnification or because the assembly itself obscures them (e.g., solder balls beneath BGAs). However, MVI is an excellent first sweep of the device that can catch many mistakes resulting from the manufacturing process.
Common Failure Modes Manually Diagnosable With PCB Visual Inspection
As the initial detection line, successful MVI can significantly reduce total inspection time. Inspectors will be responsible for a range of different defect outcomes; most, but not all of these will tend to focus on the shape, form, or appearance of certain processes rather than spotting microcracks or microvoids that can eventually propagate to more readily apparent failures:
- Solder-related defects: Soldering is an integral part of the assembly process as it joins components to exposed conductor pads of the board; it should come as no surprise that the largest family of defects is associated with the solder profile:
- Melt. Solder bonds need to exhibit a relatively lustrous appearance to indicate complete melting of the alloy and wetting on the pin for reliability. A dull solder appearance may indicate temperatures during reflow that are either too low (supported by unmelted solder balls) or too high (solder may appear porous due to oxidation).
- Fillet. Solder should flow in a characteristic shape for most packages with a toe that extends past the outward end of the pin or lead and a heel that forms between the pad and the inside arch of the pin or lead. Specific acceptable measurements will vary depending on the package and lead size. Inadequate reflow conditions may lead to visible gaps under components that trap processing fluids and are likely to experience excessive vibration that will wear poor fillets early.
- Loose or misapplied solder. The danger with unmelted or partially unmelted solder beyond the extent of where it belongs is the chance for a later reflow during processing or field operation that could lead to a short in the assembly. It is considered a nonissue when trapped or confined to a surface where reflow wouldn’t allow it to bridge connections but must be removed in all other cases.
- Vias and through-hole pads: Lifted or damaged pads can result from excessive heat beyond rated maximums. Solder is not allowed to remain within the through-hole barrels (passage during solder processing is allowed). Some solder is permissible within castellated holes, but fully-filled half holes are ruled as defects.
- Board damage: Primarily, the board should be examined for cracks and delamination that indicate early failure of the material due to thermal-related aging. Substrate cracks are forbidden, but some delamination at the board’s edge is reasonable as long as it is confined to the edge clearance region. Chipping within the solder mask layer may or may not register as a defect: solder mask voids cannot exist between adjacent conductors (ground-to-ground is valid). Similarly, cracks within the solder mask layer are permitted so long as there is no lift to the layer when checked with an adhesive.
Your Contract Manufacturer Has Its Sights Set on Quality
PCB visual inspection functions as the vanguard of quality control, able to detect a majority of manufacturing defects with a trained operator and minimal investment in supporting equipment. Even the quick pace of technology within the last several decades has yet to be able to supplant the value an experienced inspector can offer in eliminating defective or compromised devices from reaching customers. With assemblies increasing in complexity while reducing in size, the need for quality assurance is higher than ever.
At VSE, we understand how your products represent your brand, and as a team of engineers committed to building electronics for our clients, we take great pride in every design that passes through our facilities. With our team of manufacturing partners, we’ll go the extra mile to ensure your devices undergo rigorous testing before they’re approved – it’s just one less thing you and your customers need to worry about.
