PCB Inspection in SMT assembly process: ICT, AOI and AXI

While technology continues to move towards increasing levels of complexity, it is increasingly necessary to improve quality control processes before, during and after manufacturing processes. Other types of tests, such as Automated Optical Inspection (AOI) and X-ray Automated Inspection (XAI), have been added to the traditional In-Circuit Testing (ICT).

When choosing which method or combination of test methods we will use, the level of complexity of the PCB is taken into account, what is the manufacturing process that predominates in it, as well as what is the purpose of the analysis we are conducting.

 

 

In-Circuit Testing (ICT)

The ICT (In-Circuit Test) allows us to search for different type of failures such as opens, shorts, continuity tests, etc. There are two main techniques for it.

Bed of nails,Flying probe test

Bed of nails

This is the traditional exam. It seeks to generate multiple contact points in the circuit through small spring loaded pogo pins, which seen from afar maintain the similarity with a bed of nails and hence its name. Each pogo pin will make contact with a cricut node, this way a pressure is applied to the Device Under Test (DUC) and hundred of connections are simultaneously tested. Using this technique we can find component defects, also search for parameter deviation, solder joint bridging, displacement, opens, shorts, continuity tests, etc.

 

This type of test is suitable for simple PCBAs and also for mass production systems, has a low cost and is fast. However, if we try to apply it to high-density components or large-scale integration PCBs in which miniaturization has taken a leading role, we will find that there are technical difficulties that cannot be overcome. For this reason, over the years, alternative techniques have been developed for this type of test.

 

Flying probe test

 

This technique allows us to perform tests with smaller sizes, we can achieve a min test pitch up to 0.2 mm. The PCB is introduced in a test environment in which the different probes will come into contact with the pads and vias. We can analyze it searching for shorts and opens, but also the system is equipped with a camera that analyzes the shape of the components and their size. It allows us to control if elements are missing. Is also capable to allows us analyze the value of the components as resistance and capacitance, for instance. It is also possible to analyze the polarity of the elements.

 

Automated Optical Inspection (AOI)

Automated Optical Inspection (AOI)

An AOI inspection will allow us to analyze assembly and manufacturing failures. The PCB is analyzed by one or several cameras, these images are then compared through the software with a board that is taken as a parameter usually called "golden board" or with design specifications.

This type of analysis is usually performed at the end of the assembly line to ensure the final quality of the PCB. Some Pick and place machines use this technology to avoid defects in the placement and alignment of components.

 

Therefore, another fundamental aspect is that it allows us to track processes.

It allows us to monitor the assembly process and then classify and correct displacement and component assembly defects.

Usually the AOI equipment is placed in different stages of the assembly line so that the specific manufacturing situation can be monitored online and the necessary basis for the adjustment of the manufacturing technique is provided.

 

We can mention three important places to consider:

Before the application of solder paste. This will allow to control that the amount of paste applied is exact, neither more nor less. We can also avoid the lack of alignment by placing it, as well as welding bridges between pads. It is also important to configure an AOI control point Before the reflow soldering process, in this way we can ensure that the components are placed correctly before completing the soldering process.

Finally, of course, also after reflow soldering. This provides an overview of the process that allows to identify faults in both the last and previous stages.

 

 

Automated X-ray inspection (XAI)

Automated X-ray inspection (XAI)

The application of X-ray technologies to PCB inspection is a powerful tool for analyzing failures, especially for soldering analysis. It allows us to observe the inside of the solder and discover if there is a lack of filling, bubbles, etc. In PCBs where BGA technologies are present, it becomes essential because we cannot observe the solder joints made under the chip.

An X-ray inspection will allow us to observe the soldering inside and under the chip, analyzing if all the connections have been made correctly. 2D, 3D technologies are used to perform image analysis.

2D inspections look for cracks, bridges, poor alignment or also insufficient solder. This is the low cost option. There is also the option of X-ray inspection in 5D, here we compare the images obtained from the PCB with a CAD file for the differences. Using this inspection method we can make three individual cuts between the BGA and the solder balls, also enter the solder balls and evaluate in depth the connection between the balls and the pad. Therefore, using this technique our engineers may find faults that would be impossible with another technique.

 

So, what inspection method choose? ICT, AOI or XAI?

 

First, we must consider that we do not have to choose between them, but we must understand for what we will use each of them, how and when to combine them. This will depend on the level of complexity of our PCB and also on the type of fault we are looking for.

what type of failures each type of inspection can detect

It is important to be clear about what type of failures each type of inspection can detect. This table shows us this clearly.

Notice that some errors can only be detected through ICT, so this test becomes indispensable.

 

 

AOI, AXI

Therefore, our choice of options will be between using AOI, AXI or combining them. As a general recommendation we can take the graph presented here. It should be noted that a PCB may not be complex, but include BGA devices and remember the above: if we have a BGA component, only X-ray technology allows us to analyze in detail. MVI stands for Manual Vision inspection.

 

We must also bear in mind that time is money and XAI is a slow inspection technology compared to AOI, with which costs will be higher.

As a final conclusion, we must say that it is always advisable to conduct an ICT. In addition, although the cost of the application of XAI inspections is higher, there are PCBs in which we cannot stop doing so due to the presence of BGA components and also because some soldering failures only XAI is able to detect them. A combined use of all techniques will dramatically reduce process failures and scrap.