PCB assembly test design guide

After the PCB assembly is completed, the test is a very important link to detect its function. The PCB test can detect whether the function of a board is normal, and the problem of the problem is screened at the same time. Of course, testing requires a certain amount of time, but it can ensure the quality of the product, so it is worth doing this.

What is designed for testing?

DFT is an optimization method for operating and functional testing of circuit boards. This method identifies any short circuit, opening, or wrong components of error components. This test method is used to verify three main problems:
1. Is the board design accurate?
2. Is the production of board perfect?
3. Does all components, ICs and connections work normally?

Other main problems considering are:
There should be appropriate interval between components to reduce the risk of test defects
If the electrical connection between the welded film pads will decrease.
Optimization of drill size.
Wrong surface sticker installation pad size size
Test acid catcher

Why do I need DFT?

The most important goal of test design is to detect products and find problems with problems. After years of development, PCB has a thousand components, and the test space has become very limited. If any component or connector hinders the design, this will be a nightmare for manufacturers and designers.
DFT engineers and product developers have established a set of test methods to find out any inaccurate places, and produce high -quality, operable circuit boards.

What is test technology design?

There are two important test technologies here:
Bare plate test
Before assembling elements, perform bare board tests to check the connection of PCB. The following is two methods to perform this test
The isolation test tests the resistance between the two electrical connections.
Continuous test check check whether there is a road in the circuit board.
Assembly board test
The assembly board test is performed after the assembly element. This process ensures the integrity of the circuit board and the correct function of the component.

7 main test methods in PCB

1. Flying needle test
Both nude and assembly boards can be tested in the passive and active mode respectively. The probe includes a needle used to check. Test points can include passive components, such as resistance, capacitance, inductance, unpuffed or component terminals.
Advantages: It can be implemented quickly, involved widely, convenient and cheap, and high detection accuracy.
Disadvantages: Time, because the probe moves between the measuring point, it can only test the total capacitance of the packet container.

2. Online testing
Online testing is also called nail bed test method. This process includes electronic probes that are pre -installed, the preset access point under the circuit board. In this way, accurate and stable electrical connections can be established between probe and PCB. The test probe can make the current flow on the pre -determined design test point.
ICT can check short -circuit or opening, welding film defects, misalignment or lack of components. This method includes a test fixture to correctly fix the circuit board with a probe, and the test fixture to check multiple components on the circuit board at the same time. This test method saves time.
Advantages: High accuracy, can be detected in batches.

Disadvantages: Not suitable for small batch production, and no welded welded films cannot be detected. Picks are required to increase test costs.

3. Function test
It is used for quality control and ensures the expected operation of the equipment. The test parameters are provided by the customer/designer based on the design. This technology usually includes a simple switch test, and sometimes requires complex software and accurate protocols. Function test directly check the functions of the circuit board under real environmental conditions.
Benefit:
low cost. There are various functions and can be customized according to the design. It will not affect the life of the motherboard, unlike other tests to put too much pressure on it.
Disadvantages: require experienced technicians.

4. Automatic optical testing
AOL integrates 2D or 3D cameras, you can click high -resolution image and verify the schematic diagram. It is also compared with the perfect and imperfect design available in the database. This method can find out all visible errors very accurately. AO1 is used with another type of test method to ensure the correct results, such as AO and flying needles, AOI and circuit tests. It can be directly included on the production line to prevent any premature circuit board failure.
Advantages: Fatal defects can be accurately detected.
Disadvantages: Only detect surface defects, it takes a certain amount of time, detection cannot be 100%accurate.

5. Aging test
The aging test is an early inspection of the circuit board to prevent a dangerous failure after the manufacturing is completed. This method includes more than the specified operating limit to trigger the fault. This is an effective method for detecting the maximum work rated of the circuit board.
Various working conditions include voltage, current, temperature, operating frequency, power, and design related factors related to design.
Advantages: Improve product reliability and verify the functions of the circuit board under environmental conditions.
Disadvantages: External additional stress that exceeds the rated value will shorten the service life of the circuit board.

6.X ray examination
X -ray inspection detection hidden components, welding connections, BGA packaging, internal traces and barrels.
Interest: No need to check every layer of PCB. The X -ray machine can easily check the inner layer from the top of the circuit.
Disadvantages: Increasing costs requires experience and skilled technicians.

7. Exterior test
Here, the technicians are checked with the naked eye or a magnifying glass. This method can determine the alignment of the exposed elements, the lack of components, and other defects.
Advantages: Simple and basic methods.
Disadvantages: affected by artificial errors. It is impossible to detect small and invisible defects.

PCB test strategy

Width of the edge of the border
This is important for maintaining enough space along the relative edge of the cardboard. It is important. These clear edges help to hold the test machine perfectly. Generally, the standard width of the edge should not be kept at 3 mm. Generally, panel waste is also used for the same purpose.

Benchmark
The machine needs some reference points to know the accurate position of the probe. The reference point is called the benchmark point, which is located on the panel waste. If the waste has been cleared, it is located on the PCB itself. The most suitable and recommended benchmark point is in the upper left and lower right corners of the chessboard.

Excessive holes
The holes contained in the circuit board design require no shielding in order to place a probe on the edge of the holes.

Component leg
Generally, it is best to place the test probe near the component pin to achieve good solder joints, although the component pin does not need to be tested. This technology is pushed to the pad on the pad to connect any potential opening on the test point.

size
If a large circuit board is designed, the test access point should be as close as possible.

clean
Cleaning components are essential to ensure removal of excess welds. This is because sometimes the testers have to move the probe position to get better contact, and the welds that do not want can cause failure and increase the test time.

Detection point
You can introduce a trigger probe point on the ground, introduce the power rail at the bottom, and perhaps on the non -connected ground side of the PCB. This allows a fixed probe as a temporary fixture to accelerate short -circuit testing and reduce the total test time and cost.

Test access
If you can, you can maximize the test access on one side of the component. At least each network has a possible point, which is great. If you use double -sided machines, the cost of double -sided overtime tests will increase.