# Study on Application of Four-Wire and Four-Terminal Flying Probe Test Scheme

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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.

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