As PCB boards play an increasingly important role in all kinds of end products and competition intensifies, reliability requirements for PCB products keep rising. Designing a reliable board requires a rigorous PCB design process, and the first step in designing critical circuits is to check components and quality, which calls for a range of reliability tests.

1. Ionic contamination test

Purpose: to check the number of ions on the board surface and determine whether board cleanliness is acceptable.

Method: clean the sample surface with 75% isopropanol. Ions dissolve into the isopropanol and change its conductivity; the change in conductivity is recorded to determine ion concentration.

Criterion: no more than 6.45 µg NaCl per square inch.

2. Chemical resistance of the solder mask

Purpose: to check the chemical resistance of the solder mask.

Method: apply a sufficient quantity of dichloromethane to the sample surface, then after a short time wipe it with white cotton. Check whether the cotton is stained and whether the solder mask has dissolved.

Criterion: no staining or dissolution.

3. Solder mask hardness test

Purpose: to check the hardness of the solder mask.

Method: place the board on a flat surface and scratch it with standard test pencils of increasing hardness until no scratch remains. Record the lowest hardness that leaves no scratch.

Criterion: the minimum hardness shall be above 6H.

4. Peel strength test

Purpose: to check the force needed to peel copper foil from the board.

Equipment: peel strength tester.

Method: peel at least 10 mm of copper from one side of the substrate, place the sample on the tester and peel the remaining copper with a perpendicular force, recording the force.

Criterion: the force shall exceed 1.1 N/mm.

5. Solderability test

Purpose: to check the solderability of pads and through holes.

Equipment: solder pot, oven and timer.

Method: bake the board in an oven at 105℃ for one hour and apply flux. Immerse the board in the solder pot at 235℃ and withdraw it after 3 seconds, then check the area of the pads that has been tinned. Place the board vertically in the solder pot at 235℃ and withdraw it after 3 seconds, then check whether the through holes are tinned.

Criterion: tinned area shall exceed 95% and all through holes shall be tinned.

6. Withstand voltage test

Purpose: to test the voltage withstand capability of the board.

Equipment: withstand voltage tester.

Method: clean and dry the sample, connect the board to the tester, raise the voltage at no more than 100 V/s to 500 V DC and hold at 500 V DC for 30 seconds.

Criterion: no failure on the circuit.

7. Glass transition temperature test

Purpose: to check the glass transition temperature of the board.

Equipment: DSC (differential scanning calorimeter), oven, desiccator and electronic balance.

Method: prepare a sample weighing 15 to 25 mg, bake it in an oven at 105℃ for two hours, then cool it to room temperature in a desiccator. Place the sample on the DSC stage, set the heating rate to 20℃/min, scan twice and record Tg.

Criterion: Tg shall be above 150℃.

8. Coefficient of thermal expansion test

Purpose: to evaluate the CTE of the board.

Equipment: TMA (thermomechanical analyser), oven and desiccator.

Method: prepare a sample 6.35 by 6.35 mm, bake it at 105℃ for two hours, then cool it to room temperature in a desiccator. Place it on the TMA stage, set the heating rate to 10℃/min and a final temperature of 250℃, and record the CTE.

9. Heat resistance test

Purpose: to evaluate the heat resistance of the board.

Equipment: TMA (thermomechanical analyser), oven and desiccator.

Method: prepare a sample 6.35 by 6.35 mm, bake it at 105℃ for two hours, then cool it to room temperature in a desiccator. Place it on the TMA stage, set the heating rate to 10℃/min and raise the sample temperature to 260℃.