Technical information: testability of PCB design: a practical guide for beginners

When designing a PCB for the first time, it is easy for designers to devote all their energy to function realization, layout design and component selection. However, Testability (the core index of PCB design, which refers to the characteristics of the circuit board for subsequent electrical performance testing and troubleshooting), a key design element, is often ignored.
If the verification and confirmation process of the circuit board is not planned in the design stage, even if the wiring design is exquisite, the subsequent debugging, testing and mass production upgrade will become difficult.

One. Why can't DFT be ignored?

Testability will directly affect the three cores of the circuit board: yield, reliability and production cost. In fact, the cost of testing can account for up to 30% of the total production cost of PCB. Many beginners easily fall into a misunderstanding: DFT is regarded as "additional work" after the layout is completed. But the real DFT is a concept that runs through the whole design process-from schematic drawing to layout and wiring, every step considers the convenience of subsequent testing, so that the circuit board can be easily detected, measured and verified in the whole life cycle. Integrating DFT principle in advance can minimize product defects, simplify production process and avoid "jamming" in later debugging.

Second, first understand: What is DFT?

Simply put, DFT is a collection of design skills and layout decisions. Its core purpose is only one: to ensure that the key circuits of the circuit board can be easily connected for testing, parameter measurement and performance evaluation at all stages of production and use.

Specifically, DFT includes these key designs:
Reasonable arrangement of test points
Component mounting planning
Functional module division of circuit
Adaptation of automatic test method

Bottom line: Good DFT design can make the circuit board "easy to test", and then realize "good repair and good mass production".

Third, common PCB testing methods (novice must see)

In the production and verification of PCB, a variety of testing methods will be used, and each method has a clear testing purpose. Novices must understand these test methods before layout, so as to accurately match DFT design.

3.1 Bare Board Testing

Test purpose: Before the components are assembled, verify the connectivity and insulation of copper foil (to avoid congenital "open circuit" and "short circuit").
Core details: including network connectivity test and insulation withstand voltage test (Hi-pot tests); Two methods can be selected according to the output.
Flying needle test: high flexibility, no need for special fixture, suitable for prototype/small batch, slow speed;
Fixture test: fast, efficient, suitable for mass production and high cost.

3.2 Online Testing (ICT)

Test purpose: After the components are assembled, detect short circuit, open circuit and component parameter errors (such as wrong resistance of resistance welding and leakage of capacitor).
Core details: The bed-of-nails fixture (consisting of multiple test probes to realize the electrical connection between the circuit board and the test equipment) is used for testing, which is a "standard" for mass production.
Key design requirements: the size and spacing of test pads must be uniform, and positioning datum points (fiducials, special marks on PCB, to ensure test accuracy) should be reserved.

3.3 Flying Probe Testing

Test purpose: To test the electrical performance of the assembled circuit board without special fixture.
Core details: suitable for prototype trial production and small and medium batch production, with low cost and high flexibility;
Key design requirements: the test points are unobstructed, the spacing is reasonable, and the probes can easily contact the top and bottom layers of the circuit board.

3.4 Functional Testing (functional testing)

Test purpose: to evaluate the overall performance of the circuit board in the actual working environment (closest to the real use scenario).

Core details: the circuit board is powered on, and whether the system meets the design expectations is verified by exciting and monitoring signals;

Key design requirements: firmware programs should be installed, and input/output (I/O) interfaces should be reserved to facilitate signal excitation and observation (such as UART, I2C, SPI and other debugging interfaces).

3.5 Boundary Scan Test (JTAG)

Test purpose: to test the internal connection of digital circuits through scanning chain (suitable for high-density digital systems, such as single chip microcomputer and FPGA circuits).

Core details: It must conform to the IEEE 1149.1 standard and reserve a JTAG interface that can be easily accessed (it is recommended to arrange it on the edge of the circuit board to facilitate testing).

3.6 Visual/Automatic Optical /X-ray Inspection

Test purpose: identify the assembly defects of components, such as mounting deviation, solder joint virtual welding, component missing, etc.

Core details: Manual inspection is easy to make mistakes, and automatic optical inspection (AOI) and X-ray inspection are the mainstream of mass production;

X-ray inspection focus: suitable for detecting hidden solder joints (such as BGA package devices and via solder joints) and high-density circuit boards.

FOUR. Environment and Stress Testing: Ensuring Long-term Reliability

In addition to the electrical performance test, the long-term reliability of PCB also needs to be verified by "limit test"-such tests can find potential defects (such as solder joint fatigue and material aging) that cannot be detected in conventional tests.

Note to novices: The prototype stage can be selectively tested, but the mass-production design (especially in the fields of automobile, industry and aerospace) must be strictly implemented.

4.1 Common stress test types

Temperature cycle test: repeatedly withstand extreme high and low temperatures, simulate actual temperature fluctuations, and detect solder joint fatigue and material delamination;

Vibration test: simulate the vibration of equipment in operation, and detect whether the solder joint and connector are loose (suitable for vehicle-mounted and industrial equipment);

Humidity/corrosion test: simulate high humidity and salt spray environment, and detect whether the circuit board has electromigration and coating degradation;

Mechanical bending/drop test: test the physical durability of PCB (suitable for portable devices, such as mobile phones and bracelets).

4.2 Common industry standards (for beginners' reference)

There are clear industry standards for this kind of testing, and designers can choose adaptation standards according to product scenarios:

IPC-9701: Focus on the solder joint reliability of surface mount components;

IPC-TM-650: Covers detailed test methods such as thermal shock, insulation resistance and solderability;

MIL-STD-810: Military standard, suitable for PCB testing in extreme environment;

IEC 60068: International general standard, covering temperature, humidity, vibration and other tests.

Tip: The stress resistance of PCB can be improved by selecting suitable substrate, reserving mechanical gap and coating conformal coating.


Five practical skills to improve testability (for beginners to use directly)

DFT design need not be complicated. Mastering these five skills can greatly improve the testability of PCB and avoid rework later.


5.1 Standard Test Point Design

Test points must be arranged on key networks such as power rail, clock line, reset signal and data path.

Standard size: it is recommended that the diameter of the test pad be 35 mil (mil, the common unit of PCB design, 1mil=0.0254mm) and the spacing should be at least 100mil；;

Taboo: Don't place the test point under the components or in the area where the probe can't touch.

5.2 Ensure the accessibility of components

The spacing between components should not be too small, and it is recommended to be at least 100mil to facilitate the contact between the test probe and the testing tool.

Tip: Keep the mounting direction of IC chips consistent, and reduce human error during inspection.

5.3 Strengthen "Observability" and "Controllability"

This is the core principle of DFT, and novices must keep in mind:

Observability: can measure the signal state inside the circuit board (such as reserved test pads and debugging interfaces);

Controllability: It can stimulate and regulate circuit signals during testing (such as adding jumpers, leading out reset lines and enabling pins).

Tip: Add a status indicator to visually judge the working state of the circuit.

5.4 The circuit is partitioned by function and can be measured independently.

Divide PCB into several independent functional modules (such as power module, analog module and digital module), and each module can be tested independently;

Implementation method: connect zero ohm resistors in series between modules or set detachable jumpers to avoid mutual interference between modules.

5.5 Both testability and signal integrity are considered for high-speed signals.

Test points of high-speed signals (such as USB and HDMI) may lead to impedance discontinuity, which may lead to signal reflection and crosstalk;

Solution: Connect resistance protection in series at sensitive nodes, or make the test point slightly deviate from the main transmission line, giving consideration to both test and signal quality.

Six, the beginner DFT step-by-step implementation process (don't miss a step)

Beginners don't have to panic. By following this process step by step, they can easily integrate into DFT design and avoid missing key steps.

Step 1: schematic diagram stage (planning in advance)

Clear the signals that need to be monitored and regulated, and directly integrate the test points, debugging interfaces and indicator lights into the schematic diagram;

Formulate test strategy: define the verification content, test method and required equipment.

Step 2: Layout stage (strictly implemented)

According to the test requirements, standardize the size, spacing and location of test points;

Check the design: use the DFT analysis function of ECAD tool or manual audit to check whether the key test points are accessible.

Step 3: Before delivery (collaborative confirmation)

Synchronize the test scheme and DFT design details to PCB manufacturers and assembly plants;

Key confirmation: whether the design is suitable for the manufacturer's test equipment to avoid modification after production.

Seven, novice must see: DFT best practice summary

Finally, sort out the points where beginners are most likely to step on the pit and the corresponding best practices to help you avoid detours:

Communicate with the manufacturer as soon as possible to clarify the limitations of test equipment (such as the size and spacing requirements of test points);

Adapting to AOI detection: reserve positioning reference points and add clear screen printing marks;

High-speed signal design: test coupling board (test sample made at the same time with PCB for detecting impedance and signal integrity) is added;

JTAG interface: it is arranged at the edge of the board, which is convenient for testing and marked in the schematic diagram;

Don't ignore the details: even the position of a test point and the clarity of a silk screen may affect the test efficiency.

summary

For beginners of PCB, DFT is not a "plus item" but a "must item".
It is not a complex theory, but a thinking that runs through the whole design process-from schematic diagram to layout, from test planning to delivery confirmation, and more consideration of "good test" at each step can avoid the "nightmare" of later debugging, improve product reliability and reduce production costs.

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