Tests of Flexible PCB: Appearance, Solder Joints, Conductivity And Salt Spray Test

Label:Flexible PCB

Sep 25, 20242640

Tests of Flexible PCB: Appearance, Solder Joints, Conductivity And Salt Spray Test

1. Visual inspection

 

Visual inspection mainly checks the appearance of flexible PCB to ensure that there is no obvious damage and defects. Dimension inspection measures the size to ensure that it meets the design requirements.

 

2. Reliability of solder joints

 

It tests solder joints to ensure soldering is good. The test is to ensure the quality and reliability of flexible PCB to meet market demand and customer requirements.

 

3. Conductivity

 

By testing the conductive performance, you can ensure that the connection between the electronic components is good. It avoids problems such as short circuit and broken circuit.

 

There are conductive test and insulation test. The conductive test confirms whether the circuit is conductive by measuring the current and voltage. The insulation test confirms whether the circuit is well insulated by measuring the insulation resistance. Both of them can effectively discover and eliminate possible problems in the production process of flexible PCBs.

 

Anyway, for manufacturers of flexible PCB, they should establish the processes and standards for these tests. It is the key to improve corporate competitiveness and market share.


 

4. Salt spray test

 

Salt spray test is an environmental test that uses artificial simulated salt spray environment conditions to evaluate corrosion resistance of metal materials.

 

How is the test conducted? The FPC is placed in a closed environment at 35°C, with humidity >85% and a pH value between 6.5 and 7.2. It is sprayed with 5%±1% NaCl solution for 48 hours.

 

What's the judgment criteria? After the test, the conductivity must be good after 4 hours of recovery. There must be no liquid ingress in the hot pressing area, and no corrosion of the track.

 

Flexible PCB must add a gold surface sealing process to pass the salt spray test of more than 24h or 48h. Gold surface sealing uses functional substances such as chelating agents and surface film-forming agents to clean the coating, passivate active crystals, and form a protective film to slow down the corrosion of coating by external environment.

 

The longer the nickel-gold plate is stored without hole sealing protection, the worse the solderability will be. During soldering, the gold plating layer and the solder paste quickly melt together and spread on the nickel base plating layer. The tin and nickel further react to form a tin-nickel interface alloy layer. Then the soldering is completed.

 

Due to micropores in coating, the nickel base coating is easily oxidized. When the nickel surface is passivated to form an oxide layer, the surface tension decreases rapidly. It is much smaller than the surface tension of the molten solder paste. At this time, the molten solder paste cannot wet the nickel surface, which is called 'tin rejection'.

 

The nickel-gold plate without sealing cannot pass the 48-hour salt spray test. Due to the limitation of the process level, the plating layer of the nickel-gold plate inevitably has micropores, resulting in nickel exposure and impurities remaining in the micropores. During the salt spray test, these micropores provide micro-battery sites for salt spray corrosion. The more micropores, the more micro-battery corrosion sites.


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