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The necessity of PCBA cleaning

2019-11-09 16:21:17

First, the typical problem of PCBA failure caused by pollution

1. Corrosion, PCBA assembly uses iron bottom material bottom foot components. Due to the lack of solder base coverage, the iron substrate quickly generates Fe3+ under the corrosion of halogen ions and moisture, making the surface red. In addition, in humid environments, acidic ionic contaminants can directly corrode copper leads, solder joints and components, resulting in circuit failure.

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2, electromigration, if there is ion contamination on the surface of PCBA, electromigration phenomenon is very easy to occur, the ionized metal moves to the opposite electrode, and the original metal is reduced at the opposite end and a dendritic phenomenon occurs called dendritic Distribution, (dendrites, dendrites, tin whiskers), the growth of dendrites may cause partial short circuit of the circuit.

3, poor electrical contact, in the PCBA assembly process, some resins such as rosin residues often contaminate gold fingers or other connectors, when the PCBA work hot or hot climate, the residue will be sticky, easy to absorb dust or Impurities, causing increased contact resistance or even open circuit failure. Corrosion of the nickel layer on the PCB surface pad in the BGA solder joint and the presence of the phosphorous-rich layer on the surface of the nickel layer reduce the mechanical bond strength between the solder joint and the pad, and crack occurs when subjected to normal stress, causing point contact failure.

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Second, the necessity of cleaning

1. Appearance and electrical performance requirements. The most intuitive effect of PCBA contaminants is the appearance of PCBA. If placed or used in a high temperature and high humidity environment, the phenomenon of moisture absorption and fading of residues may occur. Due to the large use of leadless chips, micro BGAs, chip scale packages (CSP) and 01005 in components, the distance between components and boards is reduced, the size is miniaturized, and the assembly density is also increasing. If the halide is hidden under the component, local cleaning may cause catastrophic consequences due to halide release.

2, three anti-paint coating needs, before the surface coating, the resin residue that is not washed off will lead to delamination or cracking of the protective layer; the active agent residue may cause electrochemical migration under the coating, resulting in coating The rupture protection fails. Studies have shown that 50% coating adhesion can be increased by cleaning.

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3, no-clean also need to be cleaned, according to the current standard, the term no-clean means that the residue of the circuit board is safe from a chemical point of view, will not affect the circuit board production line, can be left on the circuit board . Detection of corrosion, SIR, electromigration, and other specialized detection methods are primarily used to determine the halogen/halide content to determine the safety of the no-clean assembly after assembly. However, even with the use of a low solids no-clean flux, there will be more or less residue, and for products with high reliability requirements, no residue or contaminants are allowed on the board. For military applications, even disposable electronic assemblies require cleaning