High-quality PCBs (Printed Circuit Boards) must conform to required parameters of their designers and have to be free from errors and failure because the success of applications PCBs serve for totally lies in the running of PCBs. Once PCBs suffer from errors, smooth running of these applications will be definitely at stake. However, PCBs are so complex in themselves that there are so many components mounted on boards and solder connections as well. Therefore, it’s extremely important and necessary to implement inspections on circuit boards.
In this article, leading inspection methods will be discussed with their advantages and disadvantages involved, which leads OEMs to pick up the most suitable inspection method based on considerations of cost and product requirement.
Visual inspection can be implemented in each step in the process of PCBA. However, equipment selection applied in visual inspection lies in positions that need to be inspected. For example, after solder paste printing and component placement, inspectors can effectively inspect large defects just with their eyes. They are capable of inspecting contaminated solder paste and missing components in less than one minute and micrometer ocular and difference Z high-degree detector can be applied for sampling inspection to inspect quality of solder paste deposition. Focusing on edge of copper pad, Z high-degree detector is adjusted to zero and then focus is set to be at the top surface of solder paste deposition so that height of solder paste deposition can be measured. The most popular visual inspection is capable of accurately inspecting reflow solder joints through observing light ray of solder joint surface reflected by ordinary prism from different perspectives. According to the size of solder joints, required amplification coefficient ranges from four-time center distance (≥1.25mm) to fifteen-time fine pitch (≤0.5mm). This inspection method is carried out based on established visual quality standard such as Mil-Std-2000A and ANSI/J-Std-001 to compare attributes of solder joints. Generally speaking, with this method applied, five solder joints can be inspected each second. Validity of visual inspection depends on inspectors’ capability, consistency of standards followed and applicability of quality of visual inspection. Inspectors must master technological requirement of each type of solder joint because each type of solder joint possibly carries 8 kinds of different fault standards and each type of PCB features more than 6 kinds of solder joints according to different component assembly. As a result, visual inspection is impractical for quantitative measurement to support effective procedure control. Moreover, this method doesn’t work on hidden solder joints inspection such as J lead components with high-density package, extra-fine square flat pack, surface array flip chip and solder joints on BGA. Through common and clear rules establishment, visual inspection is capable of providing technology that is easily implemented and this type of technology works really well on procedure development evaluation and large surface defect inspection.
Automated Optical Inspection (AOI) system
Applying multiple light sources, programmable LED library and multiple cameras, AOI system picks up images of solder joints through its irradiation from different perspectives. Leads and solder joints are like reflecting mirror in reflecting light while PCB and SMT components reflect little light. Reflecting light from solder joints fails to provide practical height data. However, image and intensity of reflecting light provide information on curvature of solder joints whose analysis can be applied to determine whether solder joints are complete or not and solder sufficient or wetting disqualified. Furthermore, AOI system is also capable of effectively inspecting solder bridge and loss or skew of components before or after soldering. Inspection rate of these systems is 30 to 50 joints per second and they feature a relatively low price that is 150 thousand to 250 thousand dollars per device. Nonetheless, AOI system fails to inspect parameters of some solder joints such as height of solder joint behind leads, solder amount within a joint, which makes capability of structure procedure control prohibited. Moreover, these systems fail to inspect hidden solder joints such as solder joints of BGA, PGA and J-type lead components that, however, are quite crucial to soldering reliability. AOI system does best in inspecting chips and gull wing shaped components whose pitch is more than 0.5mm.
First Article Inspection
Prior to volume production and assembly, PCB manufacturers and assemblers generally implement first article inspection in order to check whether SMT equipment has been correctly prepared so that vacuum nozzles or alignment issues can be avoided in the volume production, which is called first article inspection.
Ordinarily, PCB houses with high-quality service tend to assemble some pieces of PCBs in an expedited way. Then, pictures of the first few pieces will be captured or they are directly delivered to customers so that they can be tested in the practical run. In this way, quality of volume production can be smoothly carried out without any concern.
Automated X-ray Inspection (AXI)
AOI performs excellently on component pins inspection. However, inspection for pin-less components, BGA for example, has to depend on Automated X-ray Inspection (AXI). As a matter of fact, AXI isn’t the unique inspection method for such circumstance but computed-tomography (CT) scanning, specially angled optics microscope and endoscopes all can do. But AXI is absolutely the most efficient and effective inspection method.
AXI is capable of testing misalignment, missing balls and solder deposition. It observes issues through capturing pictures. Different from AOI relying on optical lightness, AXI takes advantage of X-ray to penetrate into solid things to capture their pictures. Therefore, AXI plays an important role in PCB assembly test and inspection. A sample picture taken by AXI is illustrated below.
Post Reflow Testing
Post reflow testing includes flying probe, custom testing and bed of nails.
a. Flying Probe
Flying probe is suitable for the inspection of high-complexity PCBs that require costly testing fees. Design and inspection with flying probe can be completed in one day with relatively low assembly cost. It is capable of inspecting open circuits, short circuits, and orientation of components mounted on PCBs. Moreover, it works well in identifying component placement and alignment. The only disadvantage of flying probe lies in its disability in functional test.
b. Bed of Nails
Bed of nails works right for volume manufacturing but it sticks to a relatively high engineering cost.
Responsible PCB manufacturers and assemblers all inspect boards from different perspectives in the process of assembly in order to eliminate defects and find out hindered assembly errors prior to electrical testing. Furthermore, progress and wider application of SMT (Surface Mount Technology) bring about higher requirement for inspection because solder connections applying SMT have to withstand more stress than those applying via plating technology. No matter what inspection method that’ll be selected, the key rule in this process lies in the rigorous implementation of inspection and quality management system. For example, certified by ISO 9001:2008, Kingford has been serving this business for more than ten years, which drives her to be one of the leading PCB manufacturers and PCBA providers around the world.
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