Study on Application of Four-Wire and Four-Terminal Flying Probe Test Scheme
The traditional electrical performance conduction test is to determine the open/short circuit by the on-resistance. For example, it is generally set to 20 ohms (Ω). When the entire measured line is less than 20Ω, the line-to-line continuity of the tested line can be determined to meet the requirements. However, in actual production, some defects of the PCB circuit board, such as voids in the holes, thin copper, hole copper, and separation of the innerlayers, will affect the resistance of the circuit. The above defective boards are tested by ordinary conduction tests. The results showed that PASS, after welding or application environment changes, the line resistance will change, and even lead to open circuit. These problems will inevitably have great impact on product quality.
The Problem Analysis
The traditional electrical performance test uses the four-wire two-terminal test principle. It is impossible to measure the actual resistance of the circuit accurately. The tested resistance value includes the test pin, wire and contact resistance (R1, R2), as shown in Figure 1.
Figure 1: Traditional electrical performance test.
How can we more accurately test the resistance of the circuit under test, especially when the resistance of the circuit under test is in the order of milliohm, the test pin, wire and contact resistance cannot be omitted, and four-wire and four-terminal test must be used?
Four-Wire and Four-Terminal Test
The four-wire and four-terminal test passes two pins on each test point, so that the test current and the measured voltage do not affect each other, and the measured resistance can be accurately measured as R=U/I (Figure 2).
Figure 2: Four-wire, four-terminal test.
Figure 2a: Four-wire four-terminal test pin structure.
The four-wire test needle technical requirements are as follows:
- The test needle blade is a pair of mirror structure, the tip error of the two blades is within 10 μm
- Insulation must be done between the two blades, and no contact short-circuit condition can occur during the test
- The distance between two blades must be controlled within 20 μm
- Test needle pressure needs to be adjusted to 10g–25g
How to Conduct Effective Testing
For hole breakage, voids, and insufficient hole copper thickness, the hole-to-hole test method is required. The test points are mainly set on both sides of the test hole. The test pins are placed on both ends of the hole. Theoretical values calculate the resistance of the hole and set the test setpoint for each hole directly, as in the calculation shown in Figure 3.
D = bore diameter
h = thickness of copper plating
d = hollow section diameter
L = depth of hole or plate thickness
Figure 3: Hole test schematic.
The formula, R=ρ.L/S, is the formula for calculating the conductor resistance. S is the cross-sectional area of the conductor. The unit is the square meter, ρ is the resistivity of the conductor material, the unit is ohm·m2/meter, and the copper is at 20°C. The resistivity was 1.75 × 10−8 ohm·m2/m, and the cross-sectional area represented is S=π [(D/2) 2 – (d/2) 2 ] = 2.19121 × 10–8m2.
Based on the above data, we can conclude:
Plate thickness of 1.6 mm, aperture 0.3 mm, copper thickness of 1 mil through-hole,
R = 1.75 × 10–8 × 1.6×10–3 ÷ 2.19121 × 10–8 = 1.28 x 10–3 ohms, or 1.28 milliohms
Four-Wire and Four-Terminal Test Hole Problem Analysis
If the hole copper is thin, voided, etc., the actual resistance change of this hole does not exceed 2 milliohms (Figure 4).
Figure 4: Problem analysis of four-wire and four-terminal test hole.
When the resistance deviation is more than 1 milliohm, the condition of the hole is already very poor. Especially for the current high-rise board and automotive board, the hole copper thickness must meet 100%. Figure 5 shows the customer's actual test slice results.
Figure 5: The actual test slice results.
When the resistance of the test hole is changed from 3.0 milliohms to 4.2 milliohms, there is already a single point in the measured hole with a copper thickness close to 1 micrometer.
For the same hole on the same PCB, the resistance and copper thickness changes after multiple micro-etching as shown in Figure 6.
Figure 6: Change in resistance and copper thickness after multiple microetches.
Based on the above changes in resistance and copper thickness, we found that the thinner the hole copper thickness, the greater the resistance value, and the hole resistance value is inversely proportional to the hole copper thickness.
For the via hole problem, is the other test method effective?
A couple of questions to consider: What is the effect of using the hole-adding-line test method? For the hole copper problem, can you use the hole test method?
In Figure 7, a network with a through-hole, the actual organization is R = 300 + 2 + 100 = 302 milliohms. Due to plating, etching and other fluctuations, the line resistance has a 10% fluctuation, that is, the resistance of the entire network 302 × 0.9 = 271.8 milliohms to 302 × 1.1 = 332.2 milliohms are regarded as an okay board; there is fluctuation of 60.4 milliohms. When the through-hole is thin due to the process, the hole resistance is 2 milliohms to 3 when the hole is thin. In milliohms, the resistance change is only 1 milliohm, and the actual change in line resistance at this time has reached 60 milliohms. This will directly mask the change in the resistance of the 1 milliohm of the hole. If you use the hole-adding test method, there must be a loss.
The hole-in-hole test method cannot effectively test the problem within the hole.
Figure 7: Line schematic.
What is the effect of testing with a universal/dedicated four-wire tester?
In the actual testing process, all four-wire testers or special four-wire testers test the finished PCBs. Many PCB through-holes are covered with solder mask and hole-to-hole tests cannot be performed at all.
Figure 8: Line schematic.
Due to the limitations of fixtures such as dispensing needles, slope, and probe size, it is impossible to achieve 100% coverage in the process of testing fixtures, as shown in Figure 8, if both the C and S surfaces of the through-hole are solder mask plugged holes. There are two possibilities. One is that it cannot be tested using the four-wire method. Second, even if the four-wire method is used, it is a hole test method to increase the risk of leakage.
It can be seen that testing with a universal/dedicated four-wire tester cannot effectively test the hole. The hole-to-hole test method must be used for the hole defect.
Four-Wire Four-Terminal Flying Probe Testing Machine
A Four-wire, four-terminal flying needle machine has the following capabilities:
- Different gears correspond to different resolutions
- Test accuracy and repeatability control within 5%
- The minimum test is 0.4mΩ and the maximum test is 800KΩ
Figure 9: LTD flying probe tester.
Its parameters and actual testing capabilities are shown in Table 2:
Table 2: Parameters and test capabilities.
In the process, because of poor conduction caused by residual glue and poor plating after laser drilling, it is more likely to be completely OPEN after high-temperature welding at the client end, as shown in Figure 10.
Figure 10: Blind buried hole defect.
The coil plate must test the etch fluctuation of the coil and detect the short circuit problem in the coil, as shown in Figure 11.
Figure 11: Circuit defect.
The above problems can be tested by the following methods:
This test method and software have obtained the National Computer Software Copyright Registration Certificate. The certificate number is No. 0926001.
The resistance value of each line on the board is automatically learned by the machine. The standard resistance of each circuit of this batch of boards is calculated by the software of “Board Test Analysis System Software.” The unique convergence of the discriminatory principle, you can get rid of all abnormal data in the study, the volatility can help engineers quickly understand the batch status. And according to the learning resistance fluctuations, accurate error setting values are given.
Figure 12: Board test analysis software.
With the four-wire and four-terminal flying probe test method, voids in the holes formed in the PCB due to process differences, thin copper, and poor conduction due to residual adhesive after laser drilling of the HDI board can be detected, and the circuit board guide can be solved. With the development of multilayer, high-density PCB layout, four-wire four-terminal flying probe testing will play an increasingly important role.
Jin Erbing is customer service director at Joint Stars Technology Co. LTD.