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

The Problem

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

Introduction

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 × 108 ohm·m2/m, and the cross-sectional area represented is S=π [(D/2) 2 – (d/2) 2 ] = 2.19121 × 108m2.

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 × 108 × 1.6×103 ÷ 2.19121 × 108 = 1.28 x 103 ohms, or 1.28 milliohms

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