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Printed Circuit Board Assembly Inspection Methods
Jan 05, 2017

Responsible Printed Circuit Board (PCB) manufacturers all inspect boards on different positions in the process of assembly in order to eliminate appearance defects and find out numerous assembly errors prior to electric testing and collect data for statistical process. The crucial step for this goal lies in the implementation of rigorous quality administration system. For example, ZF Electronic, certified with ISO9001:2008, conforms to all quality regulations of ISO9001, ensuring product quality in each link of manufacturing. Progress and wider application of SMT (Surface Mount Technology) brings about higher requirement for inspection because solder joints applying SMT have to withstand more stress than those applying via plating technology. Component leads with the application of SMT have to withstand more structural load, so SMT solder joints can’t be firmly soldered on board unless a large amount of solder is applied. As a result, long-term electric reliability of PCBs loading with SMT components greatly depends on structural integrity of solder joints.

 
Inspection Technology

Up to now, apart from ordinary visual inspection, multiple procedure testing technologies are applied with different prices, performances and fault coverage. Automated testing technologies include optical inspection, laser triangle inspection, X-ray inspection and X-ray layering technology. In order to implement optimal procedure testing, PCB manufacturers should be aware of pros and cons of each type of testing technology and what each method is good at. All these technologies can be classified into two categories at the highest rank: visual inspection and automated procedure testing system.

 

Visual Inspection

Visual inspection can be implemented after implementations of lots of steps 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.

 

Structure Procedure Testing System (SPTS)

Both tolerance and repeatability of visual inspection can be to a huge extent improved owing to digitization and analysis system of solder joint real-time video automatic collection. Therefore, SPTS transforms to emission light of some kind such as visible light, laser beam and X-ray. All these systems obtain information through image processing so as to find and measure defects related with quality and determines whether to maintain or to eliminate. Similar with visual inspection, SPTS doesn’t need contact between physical ground and PCB while different from visual inspection, subjectivity is eliminated from defect measurement with repeatability increased by an order of magnitude. Many systems like this system are capable of providing accurate and repeatable quantitative measurement directly corresponding structure procedure control parameters.

 

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.

 

Automated Laser Triangle (ALT) measurement

Laser measuring technology is a more direct technology measuring solder joints or solder paste deposition height and shape. This system is capable of measuring the height and reflecting rate of some surfaces when laser beam is focused on one or multiple position sensitive inspectors which are at fixed places and maintain a certain angle with laser beam. In ALT measurement, surface height is determined from position of reflecting light derived from position sensitive inspector and surface reflecting rate is figured out through beam of reflecting light. However, as a result of secondary reflection, laser beam may be irradiated on position sensitive inspectors at different positions, which acquires a mechanism distinguishing correct measurement. In addition, reflecting light beam possibly tends to be covered or shielded due to interference material along the light of position sensitive inspector, finally leading to measurement without any result. In order to eliminate multi-reflections and prohibit from shielding, this system should be capable of inspecting reflected laser beam along regulating independent light path. When carrying out massive height measurement for each solder joint, ALT system is optimal equipment for solder paste deposition and placement alignment prior to measuring component placement, providing data for real-time structure procedure control of solder paste printing such as viscosity, leakage board alignment, cleanliness, flow and squeezing velocity and pressure. Price of ALT falls within the range from 100 thousand to 250 thousand dollars based on tolerance range from 5 to 25 drops per second.

 

X-ray perspective system

X-ray layering system generates horizontal intersection through scanning or X-ray synchronously spinning with its vertical axis acute angle. An off-axis image generated on inspector tends to be equalized through one or multiple spinning so as to generate section image with the average thickness from 0.2mm to 0.4mm (On this plane, radiation beam is intersects with vertical axis). Furthermore, components at the upper and lower area of focal surface become defocusing in layering image in order to depart solder joints and other materials on the board within focal surface. X-ray layering system applies laser distance measuring equipment to draw upper surface on the board compatible with focal surface and adjusts board warpage. Then board is moved at a small vertical increment across focal surface so that different sections of the same solder point can be inspected, which is really useful for BGA and PTH joint evaluation. Double-sided PCB is vertically moved at a large increment across focal surface in order to inspect solder joints at both sides. Through changing radiation beam scanning diameter, focal surface is vertically moved so that different amplification index or visual dimension can be set. X-ray layering technology is capable of measuring attributes of all kinds of physical solder joints at different focal surfaces so that wide procedure defect coverage can be accessible. Application of calibration relationship between section X-ray image and known solder amount is capable of transforming grey readings into practical size through a regulated standard unit or metric unit. Analysis on numerous measurements allows representation and improving assembly process. For example, average solder thickness or solder amount change of solder joints allows people to observe quality rank in the process of solder paste printing and defect source. X-ray layering technology features an inspection rate of 30 to 40 joints per second while it fails to provide 100% testing coverage for some applications whose assembly period is less than 45 seconds. It can ensure 100% coverage of key components with a flexible sampling method. The price of this system is within the range from 350 thousand to 450 thousand dollars. In spite of its high price, it is cost-effective because it shrinks searching, elimination and rework time.

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