Failures in PCB Assembly and Testing of Printed Circuit Boards
PCB assembly is a multi-step process that includes surface mount technology (SMT) and through hole technology.
With through-hole technology, component pins are placed on a board and soldered using wave soldering.
Some SMD components can be placed on the underside of the board and soldered using wave soldering.
With surface mount technology, solder paste is applied to the pads of the PCB, the components are placed on them, and the board is sent to the reflow oven so that the solder paste can melt and create an electrical and mechanical connection between the component pins and the PCB.
How to Test Failures in PCB Assembly?
For failures in PCB assembly testing, the assembled printed circuit boards are placed in clamps, an electric current is applied to them, and the boards begin to work.
This group of active packages is then placed in a climate chamber, where they are exposed to temperature fluctuations to determine if this climatic stress will cause the boards to malfunction. Then the reasons for the failure are analyzed. Air Products’ experience, technology and gases can help increase profitability and equipment life, reduce defects, and optimize operating costs in IC assembly and testing.
7 Common Failures in PCB Assembly
Often after building a circuit, the reader has the unpleasant surprise of not seeing it work. Even after checking the assembly, he finds nothing wrong. However, there are many common failures in PCB assembly that compromise assembly. Some of these errors are described below.
1) Diode D1 is inverted in the assembly. Observe the position of the strip that indicates the cathode, shown in the original drawing (diagram) and how the assembler did when he performed the assembly – figure 1.
Figure 1 – Inverted Diode
2) Inversion of power transistors or SCRs. In figure 2 we show the case where the position of one of these components is inverted. Note that in the original, the component has the metal part down and the assembler did the opposite. With that the device will not work.
Figure 2 – Transistor mounted inverted.
3) Position inversion of small power transistors (SOT-54 and similar). The flat side of the component must be observed in the assembly drawing. If the transistor is flat side down when the drawing shows the flat side up, the device will not work. See this in figure 3.
Figure 3 – Low power inverted transistor.
4) Changing transistors. A different type transistor is placed in place of the original type. Confusion can occur when the component appearances are the same, as in the case of BC548 and BC558, for example. They are different and the exchange will prevent the device from working.
Figure 4 – Changing types. BC548 and BC558 are different.
5) Connection connected in the wrong place. A wire soldered at the wrong point on a terminal board or bridge can compromise assembly. See the example in figure 5.
Figure 5 – Welded wire in the wrong place.
6) Wrong component connection. Observe in figure 6 that while the resistor in the original drawing is connected to the middle terminal (base), in the assembly done, the resistor was connected to the emitter terminal (on the right). This type of error compromises the operation of the device.
Figure 6 – Wrong component connection.
7) This failures in pcb assembly is very common in bridge assemblies, which are more elementary assemblies, where short circuits between component terminals can occur. Note that the resistor terminals in the figure must be separated. If one touches the other, a short circuit may occur, compromising the device’s operation;
Figure 7 – Component terminals touching each other.
How to Find Defects in Electronic Boards?
Knowing how to find defects in electronic boards is one of the most important virtues of automakers of excellence. In practice, the constant search is to prevent these errors from happening.
However, when there is efficient management of checks, they can be identified early on and avoid being replicated to a large number of parts.
Recently, we talked here on the blog about how to make mistakes is expensive for the industry . In this article we highlight the importance of keeping processes well aligned to avoid:
Stop production – A production line where failures in pcb assembly has been identified needs to stop immediately. Depending on the volume and rhythm of the assembly, this can be a huge loss.
Divert professionals from the original role – Finding the cause is the first step in solving the problem, but it is not always evident. Therefore, it is possible for some professional to dedicate himself exclusively to this, which is not interesting for the smooth running of the factory.
Avoid tarnishing the company’s reputation – Worse than the previous two options, is when the defect is not identified and reaches the consumer. Then the consequence can represent a damage to reputation and a drop in sales.
How to Find Defects in Electronic Boards in Practice?
According to our experience, one of the biggest sources of error in the assembly of electronic boards concerns the application of solder paste. Historically, approximately more than 70% of defects have come from this stage of the process.
This is a critical moment, as the pads that should receive solder paste “match” the stencil openings.
Despite being a very simple process, it requires a lot of care with the equipment, the paste and the technique used. But solder paste is not the only problem that can occur. Learn in practice how to find defects in electronic boards:
Where’s the Defect?
This is the first step to learn how to find defects in electronic boards.
For that, you need to know where to look. Solder paste is a good clue, but it’s not the only option. It is interesting that your company adopts an error investigation methodology, in which each item can be checked separately.
This checklist will depend on how the assembly process works, but there are some items that are essential:
MACHINERY
Check that all equipment is functioning correctly. Predictive maintenance and equipment calibration cannot be neglected, otherwise it becomes more difficult to know how to find defects in electronic boards.
If you are absolutely sure that everything is working perfectly, you can follow other leads.
HUMAN FAILURE
Some assembly steps depend on human intervention, so it is essential to align all the details with the team even before the work begins. When this is not done, some errors can occur due to communication or understanding failure.
That is why the process sheet is an indispensable tool in an excellent assembly line.
DESIGN ERROR
It may be that the error did not happen in the execution, but in the conception. In this case, the card layout needs to be changed entirely.
That is why the help of experienced professionals and the execution of prototypes before final assembly is essential.
The correct conception of the stencil design is of paramount importance to prevent defects such as short, insufficient or excess solder in the pads in the step of the solder paste application process.
Testing Techniques and Equipments
Since there are many sources of failures in pcb assembly you may be asking yourself, “How do I know my card is defective? Are there equipments or techniques that help to know how to find defects in electronic boards? ”.
Yes, they exist and are fundamental to not allow the plate to leave the assembler showing a defect. See what they are:
BOUNDARY SCAN
It is a method used to test interconnectors on printed circuit boards or sub-blocks within an integrated circuit. Boundary scan is also used to observe states of integrated circuit pins, measure voltage or analyze sub-blocks.
FUNCTIONAL TEST
It is made in the finished product, that is, exactly how it will be used by the final consumer.
It is extremely efficient because it mimics a real situation and is completely contextualized.
When this technique is not used, it is possible that, when implanting the plate in the product, there is some external influence that makes the operation difficult and makes it more difficult to identify the defect.
Electrical Test with Nail Bed
It is a test done with a template containing needles. In this equipment, measurements are made at specific points on the electronic board. To apply this technique, it is necessary that the company has adequate equipment and specific hardware for the function.
There is also the possibility of making small measurements with traditional equipment such as multimeters and oscilloscopes.
How to Assembly Electronic Boards
Knowing how to assemble electronic boards is a vocation of specialized companies.
As it is a dynamic, highly precise task that requires a specific degree of knowledge, being in constant improvement is a need that makes the task even more complex.
Not to mention the purchase of equipment to keep the production line up to date, team training and input management.
That is why many companies have chosen to hire PCBMAY instead of internalizing their processes and practicing self-assembly management. It is simpler, it can be cheaper and, most importantly, it has more effective results.
Below, we list 10 steps that illustrate how to assemble electronic boards with excellence. This does not mean that all projects will go through the same steps. On the contrary, good assembly depends on the customization of the service and the individual assessment of the needs of each project. However, in order to achieve excellence, it is important to have knowledge about all possibilities and only then make the best choices for assembling the product.
- PLANNING
It all starts with planning, but in the case of how to assemble electronic boards, this becomes even more important. It is clear to realize that, once assembled; hardly a plate can be repaired or redone. The chance of an error in the repair occurring that leads to the need to carry out the assembly from scratch is very great.
Therefore, one of the most important conducts for PCBMAY is the initial alignment with the customer regarding all the needs of the board. The goal is that the end result is exactly consistent with the expectation and that there are no setbacks related to the communication failure.
- PROCESS SHEET
Still talking about communication, the process sheet is one of the main secrets on how to assemble electronic boards without errors. From this initial alignment, all instructions are passed to this document, which will be the assembly guide.
To be successful, the process sheet needs to be standardized and understandable by all team members. The main advantage is that of shielding the production line against unforeseen events, such as changing an employee, for example. With an explicit process sheet, all instructions are clear and the assembly is standardized.
- RECEIPT OF COMPONENTS
So far, you’ve seen that knowing how to assemble electronic boards doesn’t just depend on the process itself, right? This is one of the steps on which not everyone imagines relevance. However, the receipt of incorrect (or ineffective) components is capable of generating serious consequences for the final product, such as the replacement of components or the placement of parts in poor condition.
When receiving materials, they must be checked, counted, identified and labeled, inserted in the system, inspected and, finally, sent to the stock. This process is the basis so that, when the kits for the assembly itself are separated, no error occurs and everything goes as planned.
- COMPONENT REGISTRATION
When you know how to assemble electronic boards accurately, all items go through the registration of components . At that time, items are separated with codes (IPN). These codes are generated by the Engineering sector, with the assistance of the client (responsible for the project).
In addition to organization, this prevents confusion between components with very similar characteristics. If the identification is made according to the characteristic, not the code, there can be confusion and a disastrous result.
- PROGRAMMING THE MACHINES
With the previous processes well aligned, programming the machines becomes a simple and precise task. The employee just needs to know the equipment well and how to assemble electronic boards.
Another important point is to make sure that the calibration is correct and that the necessary inputs for assembly have been placed, such as the components themselves or fluids for the application of solder paste, for example.
- INSERTING COMPONENTS
Having done all the previous steps, you need to know how to assemble electronic boards, in fact.
Of course, the machines will do the job. The automatic insertion equipment for SMD components is extremely fast and with impressive precision.
However, if they are not well programmed and guided by a well-defined process, this efficiency is useless.
The checking and inspection of assembly configurations can be done by offline software. It provides data and images that allow the performance analysis, dimensions, positioning and polarities of components, positions and speeds of collection and insertion and other assembly characteristics.
Thus, if an error is occurring, such as the lack of a component, for example, it is possible to identify and correct it immediately.
- APPLICATION OF SOLDER PASTE
The application of solder paste is a very sensitive process and the biggest source of errors when it comes to assembling electronic boards. It is at this moment that the PCI is inserted into the equipment and the pads that must receive solder paste “match” with the openings of a metal plate called a stencil. A poorly applied paste certainly means a product with a greater chance of inconsistency.
Some precautions are essential. Among them, a good quality stencil, PCI well supported, solder paste within validity and in good conditions for homogenization and cleaning of equipment.
- REFLOW OVEN
The application of the solder paste causes the components to be fixed, but not soldered on the plate. Only high temperatures in a reflow oven make this component insertion definitive.
Each plate needs the definition of a thermal profile, that is, a configuration involving the characteristics of the product, temperature and time. This profile is developed using equipment called a thermal profile tracer.
- X-RAY INSPECTION
The plate was assembled respecting all necessary care, but how do you know if it is really correct? X-ray inspection is an effective way to find errors (even those invisible to human eyes). This process is not always necessary, as most of the time traditional tests are sufficient. However, having this resource available adds value when it comes to products that demand maximum precision.
X-ray inspection is able to identify inconsistencies such as:
- Solder volume – as we already mentioned, the excess solder for compromising the operation of the plate. If this is the problem, the x-ray is able to identify.
- Ball voids – Known as “voids”, these voids can be tolerated in small quantities, but if they are above the limit, they must be identified on the x-ray plate inspection and repaired.
- Short circuits – They can also be identified by x-ray and occur mainly due to improper joints between balls.
10. LASER ENGRAVING
The laser engraving is a step that ensures the traceability of electronic boards. Every product needs to be identified, at all stages of the production process, before being sent to the customer. When there is no laser engraving, this identification is done with common labels. The problem is that, over time, they can come loose or lose the ink to the point of being unreadable.
With laser engraving, data such as:
- Production date;
- Name of the automaker,
- Customer and product;
- Fabrication order;
- Inspection data.
Conclusion
In any type of electronic assembly, great care must be taken with the positions and connections of the components. What we saw is valid for terminal bridge assemblies, but the same also applies to any other type of assembly. In printed circuit board, contact matrix, etc., the reader must always be attentive to the smallest details of the project.