Method of testing in PCB assembly process
Method of testing in PCB assembly process

The SMT inspection methods mainly include manual visual inspection, solder paste thickness gauge detection, automatic optical inspection, X-ray inspection, ICT test, flying probe test etc. because the content and characteristics of each process are different, the detection methods used in each process also different.
In PCB assembly factory at Shenzhen China, manual visual inspection, automatic optical inspection and X-ray inspection are the three most commonly used methods in the surface assembly process inspection. ICT testing can be performed as both static and dynamic testing.

Manual visual inspection
This method requires less investment and does not require test program development, but is slow and subjective. Due to the mentioned shortage of visual inspection, in current SMT production line, it is rarely used as the main quality inspection method,  mostly used for repair and rework.

With the miniaturization of components, the finer pitch and the further improvement of assembly density, direct visual inspection becomes more difficult or impossible. For the current 0201 and 01005, it is impossible to judge the soldering quality status with the naked eye.
Therefore, most of the visual inspections require various optical magnifying glasses and special optical instruments. Typical products include the German company ESA's Microscope, OK's VPI, and related products from Christie's.
On the one hand, the German Scope company's Microscope and OK's VP can complete the conventional SMT solder joint inspection, and can to some extent realize the hidden solder joint inspection of QFN, BGA. This is what visual inspection cannot do;
On the other hand, it is equipped with a measurement function and a full-view function to promote the application of automatic optical inspection methods in process of research and development and defect diagnosis.
With the reduction in the size of component packages and the increase in the density of circuit board patches, the difficulty of SMA inspection has become increasingly difficult. Manual visual inspection has become inadequate, and its stability and reliability are difficult to meet the needs of production and quality control. Motion detection is becoming increasingly important.

Using automatic optical inspection (AOI) as a tool to reduce defects can be used to find and eliminate errors early in the assembly process and to achieve good process control.
AOI adopts advanced visual vision system, new light feeding method, high magnification and complicated processing methods, which can obtain high defect capture rate from the surface at high test speed.
The AOI system can inspect most components, including rectangular chip components, cylindrical components, electrolytic capacitors, transistors, SOP, QFP, etc., and can detect missing components, incorrect polarity, placement welding offset, excessive solder or Insufficient, solder joint bridging, etc., but can not detect circuit errors, and at the same time can not detect the invisible solder joints.

AOI's position on the SMT production line.
There are usually three types of AOI equipment on the SMT production line.
The first is AOI which is used to detect solder paste failure after screen printing, and is called screen-printed AOI.
The second type is AOI which is used to detect device placement failure after placement, and is called post- placement AOI.
The third is AOI which is used to detect device mounting and soldering failures after reflow process.
AOI detection is one of the most commonly used inspection methods in SMT.