The release of high-quality printed circuit boards and assemblies is impossible without PCB assembly testing at all technological operations. Control is performed by various methods – visual, electrical, functional and others. In this case, electrical control is the most organic for printed circuit boards, since the main purpose of the boards is to transmit electrical signals. When designing the stage of electrical control, it is necessary to decide which method of electrical testing should be chosen, in which range to vary the test parameters.
What is PCB Inspection?
Electrical inspection of printed circuit boards (PCB) and printed circuit assemblies (PCBA) is an important production step. Its purpose is to check the integrity of the PCB, which involves checking for open circuit, short circuit, correct topology.
However, this value often does not cover defects that arise during the operation of the PCB and defects associated with an increase in the signal transit time in RF boards. In these cases, lower values are selected as the upper limit.Measuring the true ohmic resistance values of circuits requires the use of a four-probe system, which is technically difficult to perform without sacrificing the performance of testers. The use of impulse current loads to identify weak elements in connections is also associated with a loss of performance, since the mode of such control includes several sequential stages: coming into contact of the probes – measuring the resistance of the circuit with storing its value – loading the circuit with a large current impulse – measuring the resistance and comparing it with the previous value – leaving the probes from the contact.
How to Maintain Accurate PCB Assembly Testing?
In addition, for PCB assembly testing with currents, the pressure of the probes on the PCB pads must be greater than for conventional testing. This can damage the contact surfaces. For isolation monitoring, an insulation resistance threshold of 100 MΩ is generally used. Control at a higher level slows down the testing process, at a lower level it threatens to miss insulation defects. Revealing weak points of insulation with high voltages is limited by low electrical strength of air.
For testing insulation for a limited time (about 1 s), a voltage of at least 500 V is required, but at the same time, electrical corona begins from the sharp edges of probes or PCB conductors, and therefore false results of insulation testing are possible.
The most technically challenging component of electrical control is the contact system with the board under test. There are several methods of electrical contact: manual, using an adapter (contactor), movable probes, “flying dies”.
How to Execute Manual PCB Assembly Testing?
With the manual PCB assembly testing method, the operator sequentially tests all circuits with probes, but according to statistics, the percentage of missed defects reaches 25%. During testing with the adapter, all probes placed on it are in contact with the test points, and the control speed is determined by the speed of the switchable keys.
Providing high productivity, this method, however, requires significant conversion costs. Equipment with movable probes is characterized by ease of changeover, but its performance is low. The trade-off between versatility and performance is provided by the flying matrix method.
This test means testing the individual components on the board, and testing the component excludes the influence of the branch circuits. In-circuit control is subdivided into analog and digital. Analog control usually checks: the presence of short circuits and open circuits, the ratings of discrete components, the presence and correct installation of microcircuits.
The choice of equipment and contacting method for in-circuit control depends on the requirements and capabilities of the PP and PU manufacturer. For example, the method of clips and probes (manual testing) is universal, inexpensive, but requires a lot of time and relatively high qualifications of personnel, so it is usually used for one-off production and repair. The adapter method involves making a test contactor for each product.
It is possible to use the method of moving probes, which makes it possible to reduce the cost of changeover, however, special requirements are imposed on the equipment to control the assembled modules, and the productivity of this method, as already mentioned, is low. The scorpion method is used to control PU with high components.
Functional control: This control checks the performance of the assembled product, while performing the following tasks:
- Supplying the supply voltage with the ability to change it in automatic mode from the minimum to the maximum permissible for this product
- Supply of digital and analog input signals in a wide range of frequencies and voltages
- Measurement of parameters of output signals
- Emulation of loads
- Data exchange between the tester and the tested device
- Processing of measurement results and their output to the monitor and printer in a user-friendly form
- Accumulation and processing of statistical information
- During functional testing, contact with the workpiece is usually made via the edge connector.
Basic Technoloyies of Electric Control
Checking the boards using an adapter consists in installing them on test adapters, followed by checking the connectivity (i.e., detecting short circuits / open circuits with low voltage, about 10 V) and monitoring the insulation for leakage and high voltage breakdown (up to 500 AT). The presence of test probes in vias, physically located on the same PCB track, makes it possible to accurately localize breaks.
Checking even the most complex board using this method takes only a few seconds.
The bottleneck in terms of versatility in such installations is the adapter part itself. The cheapest (but also the least universal) is the adapter design in which the wires go directly from the probe holders to the measuring part. In this case, the transition from checking one board to another is a long and laborious process.
A more expensive and universal solution is when the base of the adapter part has a connector through which a replaceable element is connected to the measuring module. This design is in greatest demand among Russian manufacturers due to the wide range and low serial production.
PCB Assembly Testing via Flying Probes
Test equipment with movable probes is optimal for the production of high-density electronic modules of a wide range. Installations of this type have several probe heads with drives along the X, Y, Z axes, which alternately, according to a pre-developed program, contact the board. When contacting, a signal is applied and measured.
To control this method, no additional adapters are required, and to switch from one board to another, you just need to change the testing program. The absence of the need to manufacture test adapters, the development of a program for moving the probes by translation from a CAD system, significantly reduce the time for preparing a test program and moving from one board to another. However, this method does not provide high test performance.
To drive probes along the Z axis, a solenoid is usually used because of its low cost. It is important that the probes have the ability to control the pressure – by setting the optimal probe delivery pressure; you can hold PCB assembly testing without leaving probe marks on its surface.
One of the main indicators of testing systems is the minimum step with which the system is able to test. To provide it, the extreme row of probes is located at a short distance from the matrix border, which allows testing circuits with closely spaced contact pads. In this case, the matrices converge close to each other.
An important advantage of systems with flying matrices is the ability to simultaneously test several circuits on the board (for simple PCBs). In this case, testing is performed in parallel using two measurement systems. All this leads to a speed of up to 70 test / s, which is 10 times higher than similar characteristics of systems with moving probes. And the use of automatic loaders allows the use of installations of this type in three shifts, providing the required productivity and return on investment.
Electrical control is based on the presence of the “probe-board-conductor-probe” or “probe-board-conductor-component-board-conductor-probe” system. And the most responsible part of the control settings is the test probe itself, since the reliability of the information obtained as a result of testing depends on the quality of contact. Test probes for the adapter method usually consist of a spring-loaded contact piece and a cartridge.
The contacting part is designed to ensure high-quality contact with the PCB and can have various shapes (crown, needle, funnel, etc.), which are determined by the type of contacting object: via, pin components output, specially prepared test pads, etc.
Moving probes in Flying Matrix systems or moving probes are usually manufactured with a universal tapered tip. It is important that the pressure of the probe on the board is adjustable (for testing boards of various thicknesses and rigidity, its value ranges from 10 to 150 g). To reduce the likelihood of false errors, it is necessary for the probe drive along the Z axis to have feedback.
Basing of Tested Blocks
An important part of the equipment for electrical control of PCB and PU is the basing and installation system, since false errors occur due to misalignments between the coordinates of the board under test and the probes. In adapter installations, as a rule, pin or stop positioning is used. This basing method provides the required accuracy for PCB assembly testing, since all probes are stationary.
When using technologies with moving probes (or probe arrays), this basing method is not applicable, since the positioning error of the carriage is added. In this case, automatic basing by test coupons or fiducial marks is applied. If test coupons are used, then their coordinates are measured electrically, and the base point of reference is calculated from them.
For a more accurate and quick finding of the origin of coordinates, the method of optical measurement with the help of video cameras is used – using software, its center is determined from the image of the fiducial mark and coordinates are calculated. This makes it possible to compensate for angular rotation and displacement of the workpiece relative to the base of the machine, as well as to reduce the accumulated error in the case of a group workpiece (when recognizing benchmarks on each tested board in a group workpiece).
The Role of CAD Software in PCB Assembly Testing
For the electrical control of a PCBassembly testing, two types of tests must be carried out: for continuity and for disconnection of circuits. With N circuits tested, the number of tests for the continuity test is N, and for the disconnection test N (N-1) / 2. As you can see, in the general case, the test for disunity requires a much longer testing time, so an important part of testing systems is software that allows you to optimize the test program.
During testing, the initial data is information about the PCB layout (list of nets, data about closely spaced nets, etc.). The netlist can be created in three ways: using a reference board, from Gerber format data, and borrowing from CAD data.
In the first case, the netlist and the program are created by self-learning on the reference board. This method is only applicable to Flying Array and Moving Probe technologies. Its advantage is that test preparation takes very little time, since no board analysis is required.
However, the method also has disadvantages: firstly, it is difficult to obtain a reference board, and secondly, if the reference board contains a defect (a defect on the film, errors in the Gerber file), it cannot be detected during the production process. Thus, this method is quite complex and depends to a large extent on the ability to obtain a reference board.
In the case of using Gerber data, the netlist is recreated using software. The advantages of this method are that there is no need for a reference board and, in addition, the ability to detect systematic defects in the PCB. Its disadvantages: the time required for the analysis of Gerber data ranges from one to several tens of minutes; it is impossible to determine the defects introduced at the stage of preparing information for production (error of the operator of the automated process control system). The method is much simpler than the previous one, but it does not allow detecting all possible errors.
The best method for obtaining a netlist is to use CAD data. Its advantages: no need for a reference board; the ability to detect all types of defects that arise both during the manufacturing of the board and at the stage of information preparation.
Disadvantages: considerable time to prepare the netlist from CAD data (up to tens of minutes); each CAD has its own data format, which often does not allow 100% testing with complete information. This method is the most reliable, but requires a description of the format used in the CAD. Standard formats containing all the necessary information for testing are just beginning to be implemented in production (ODB ++).
The prerequisites for the selection of electrical control equipment should be performance, range of products, board complexity class. If low productivity is required, it is recommended to use installations with movable probes. For high-performance multi-product industries, flying matrices with automatic loaders are most justified. For large-scale production with a small nomenclature, it is recommended to use the adapter method. When choosing equipment, you should also pay attention to the supplied software.
PCB Assembly Testing Conclusion
The growing complexity of manufacturing electronic boards, in the face of a decrease in margins and an increase in production costs, imposes well-defined project rules that are not limited to pure circuit functionality, but provide for test procedures with a broad spectrum of coverage. That is why it is very important to have a comprehensive PCB assembly testing knowledge,what’s more,PCBMay make assurance 100% electrial test for PCB and assmbly.