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PCBA Cleaning Effect Evaluation
Sep 14, 2017

First, PCBA pollution

Contaminants are defined as any surface deposits, impurities, slag and adsorbent that reduce the chemical, physical or electrical properties of PCBA to unqualified levels. Mainly in the following areas:

1, constitute the PCBA components, PCB itself pollution or oxidation will bring PCBA board pollution;

2, the production process of flux residues generated, but also the main pollutants;

3, the welding process generated by the hand mark, chain claws and fixture mark, and other types of pollutants, such as plugging glue, high temperature tape, handwriting and dust, etc .;

4, the workplace dust, water and solvent vapor, smoke, tiny particles of organic matter, and static electricity caused by charged particles attached to the PCBA pollution.

Second, the harm of pollution

Contamination may directly or indirectly cause PCBA potential risks, such as:

1, the organic acid in the residue may cause corrosion to PCBA;

2, the residue of the ion in the power process, due to the potential difference between the solder joint caused by electromigration, the product short circuit failure;

3, the residue affect the coating effect;

4, after time and ambient temperature changes, the emergence of coating cracks, Alice skin, which cause reliability problems.

Third, the typical cause of PCBA failure caused by pollution

1, corrosion

PCBA assembly is the use of iron substrate at the end of the bottom of the foot components, iron substrate due to the lack of solder at the end of the cover, in the halogen ions and moisture corrosion quickly produce Fe3 +, so that the board red.

In addition, in the humid environment, with acidic ion pollutants can also directly corrosion copper wire, solder joints and components, resulting in circuit failure.

2, electromigration If the PCBA surface ion pollution exists, prone to electromigration phenomenon, the emergence of ionized metal to the opposite electrode movement, and in the reverse side to restore the original metal and dendritic phenomenon called dendritic distribution (Dendrites, dendrites, tin whiskers), dendritic growth may cause local short circuit.

3, electrical contact bad

In the PCBA assembly process, some resins such as rosin residues often pollute gold fingers or other connectors, the PCBA work in hot or hot weather, the residue will produce sticky, easy to adsorb dust or impurities, causing contact resistance increase Big or even open to failure. The presence of corrosion in the nickel layer of the PCB surface pad and the presence of the phosphorus-rich layer on the surface of the nickel layer reduces the mechanical bond strength between the solder joint and the pad, and cracks when the normal stress is applied, resulting in point contact failure.

Fourth, the need for cleaning

1, appearance and electrical performance requirements

PCBA pollutants the most intuitive effect is the appearance of PCBA, if placed in high temperature and humidity environment or use, there may be residual moisture absorption phenomenon. Due to the large number of components in the use of leadless chips, micro-BGA, chip-scale package (CSP) and 01005, components and circuit board between the narrow, miniaturization, assembly density is also growing. If the halide is hidden below the component, local cleaning may cause catastrophic consequences due to the release of the halide.

2, three anti-paint coating needs

Prior to surface coating, no resin residue that has been washed off can cause the protective layer to delaminate or crack; the active agent residue may cause electrochemical migration beneath the coating, resulting in failure of the coating rupture protection. Studies have shown that 50% coating adhesion can be increased by cleaning.

3, no cleaning also need to clean

In accordance with current standards, the term "no-clean" means that the remnants of the circuit board are safe from a chemical point of view and do not have any effect on the circuit board production line and can be left on the circuit board. Detection of corrosion, SIR, electromigration There are other specialized means of detection is mainly used to determine the halogen / halide content, and then determine the cleaning of the assembly after the completion of the safety of the assembly.

However, even if the use of solid-free low-clean cleaning flux, there will be more or less residue, for high reliability products in terms of the circuit board is not allowed any residue or contaminants. For military applications, even the disposable electronic assembly is required to be cleaned.

Fifth, cleanliness requirements

The electronics manufacturers are faced with the degree of cleanliness required to produce reliable hardware. "Clean enough to be clean enough," the problem to more and more narrow wires and lines to bring more challenges. Acceptable cleanliness in a field in industry (such as a toy after a SMT wave soldering) may be unacceptable for other areas (such as flip chip packaging).

Many craft specialists may not be well aware of the cleanliness, the challenge still exists in the case of some or some of the long-term reliability issues associated with the residue, or whether the functional impact of the residual on the hardware is determined.

Need to consider the following factors:

1, the terminal use of the environment (aerospace, medical, military, automotive, information technology, etc.)

2, product design / service cycle (90 days, 3 years, 20 years, 50 years, shelf life of +1 days)

3, the technology involved (high frequency, high impedance, power supply)

4, the failure of the phenomenon and the definition of the terminal products 1,2,3 level corresponding to the product (for example: mobile phone, heart rate regulator).