SEMIPCB

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 inspection of PCBA boards always include: Visual inspection, AOI, X-Ray, FCT Test, ICT Test, etc. The crucial step for this goal lies in the implementation of rigorous quality administration system. 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.

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.

X-ray perspective system

X-ray perspective system transmits a beam of radiation from a point light and vertically through PCB. With radiation passing through PCB, solder joints make radiation intensity weakened by a much higher extent than that caused by other materials on the board. These intensity changes on radiation energy are transformed into digital X-ray image at a 256 grey scale. An X-ray image at grey scale of a solder joint is as a matter of fact a density diagram indicating solder joint thickness, distribution and internal integrity. On single-sided PCB, X-ray perspective system is capable of accurately inspecting defects of solder joints such as those on J-type lead components, gull wing-shaped components, passive chips and transistors with a small scale. These systems also can inspect escaped components and inverted tantalum capacitor. When it comes to double-sided boards, X-ray perspective image of solder joints may be overlapped with that of solder joints at the bottom, which makes it difficult for accurate measurement.

FCT Test

FCT Test refers to functional testing. Typically, the functional test is performed during the last phase of the production line.[1] This is often referred to as a final quality control test, which is done to ensure that specifications are carried out by FCTs. The process of FCTs is entailed by the emulation or simulation of the environment in which a product is expected to operate. This is done so to check, and correct any issues with functionality. The environment involved with FCTs consists of any device that communicates with an DUT, the power supply of said DUT, and any loads needed to make the DUT function correctly. FCTs uses customer specific connectors, rather than a test point on the PCB. Functional tests are performed in an automatic fashion by production line operators using test software. In order for this to be completed, the software will communicate with any external programmable instruments such as I/O boards, digital multimeters, and communication ports. In conjunction with the test fixture, the software that interfaces with the DUT is what makes it possible for a FCT to be performed

ICT Test

ICT Test refers to In-Circuit Test. ICT is a powerful tool for printed circuit board test. Using a bed of nails in-circuit test equipment it is possible gain access to the circuit nodes on a board and measure the performance of the components regardless of the other components connected to them. Parameters such as resistance, capacitance and so forth are all measured along with the operation of analogue components such as operational amplifiers. Some functionality of digital circuits can also be measured, although their complexity usually makes a full check uneconomic. In this way, using ICT, In-Circuit Test, it is possible to undertake a very comprehensive form of printed circuit board test, ensuring that the circuit has been manufactured correctly and has a very high chance of performing to its specification.