Why Clean? Print E-mail
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Written by Harald Wack, Ph.D.   
Thursday, 31 March 2011 13:52

The PCB design needs to consider capacitive surface effects.

Let’s be honest: If OEMs and EMS did not have to clean assemblies, they wouldn’t. No one wants to invest the capital or commit operational funds and manpower unless absolutely necessary. So why has cleaning become such an important part of many electronics production lines? 

Effectively removing contamination is one step toward guaranteeing the long-term reliability of mission-critical circuit assemblies. While printed circuit boards in ordinary consumer products are not intended to last, high-reliability products in the military, aerospace, communication, medical and automotive industries demand a long life, and guaranteed.

Several types and sources of contamination must be removed from PCB assemblies. Possible impurities can be a result of bare board manufacturing processes, component manufacturing processes or assembly processes. Contamination is often present on the incoming bare boards and components. These include plastic, metal and fiberglass particulate residues from drilling and machining, as well as salts from plating and etching operations.

Furthermore, the process that ensures the long-term solderability of bare boards produces contamination that can compromise assembly performance. A common method to ensure extended shelf life of a bare board is hot-air solder leveling (HASL). In this process, the potential primary sources of contamination are the ionic and polyglycol content of the applied flux and impurities (i.e., oil, rust, etc.) in the hot air blown onto the boards. Fluxes are very aggressive and can greatly increase the ionic contamination level. Polyglycols in the flux are particularly troublesome, as they can penetrate improperly cured coatings and leach out.

When components are manufactured, contamination can result from metallization baths, rinse water, de-flashing chemicals, mold release agents and flux residues. Finally, during assembly, solder paste residue, reflow condensates, outgassing, manual welding and handling residues (fingerprints) can create problems. In particular, solder pastes occasionally produce small solder balls, which can cause serious circuit reliability problems.

In the assembly process, it is recommended to check any new lot of bare boards and components for ionic and non-ionic contamination levels generated by dust and oils, etc. This regular procedure reduces post-assembly surprises. Additionally, if not rinsed properly, the very cleaning agent designed to remove soils generated during the assembly process can itself generate contamination. Even the water used in washing or rinsing processes can leave residual impurities on boards if not effectively dionized.

How does contamination affect the reliability of circuit assemblies? Assemblies are cleaned to remove those contaminants that could be corrosive to joints and components and ultimately result in circuit failure. These failures can have several root causes, including electrochemical migration, dendritic growth, and electrical leakage currents. None of these phenomena is good for reliability, as they will eventually cause failure.

Contaminants left on boards can directly short-circuit components or absorb moisture from the atmosphere, thereby reducing resistance between leads and promoting electrochemical migration and dendritic growth. In simple terms, the combination of humidity and contamination can result in conductive electrolytes. Add stress voltage, and electrochemical migration and dendritic growth can occur. Contaminant sensitivity increases when high-impedance circuitry is used. Thus, while a tiny electric leak across a low-resistance component may have a negligible effect on a circuit’s performance, the same leak could be disastrous across a high-
resistance component.

One of the latest trends is high-frequency (HF) circuits. This circuitry requires clean surfaces to ensure functionality; and of course, correct functioning largely determines end-product reliability. Conventional circuits with high ohmic components are increasingly sensitive to climatic disturbances due to small current leakages interpreted as signals when reaching the same current level. High-frequency circuits between 30MHz and 5GHz are even more sensitive to climatic disturbances. To ensure signal integrity, two factors are necessary: high ohmic resistance and stable impedance. This means that in the PCB design, capacitive surface effects need to be considered. Further, the environments in which these HF circuits operate are full of humidity and harmful gasses. This can result in failure due to the interaction between the flux residue and environmental contaminants, if the assembly has not been properly cleaned.

With present high-end assemblies, corrosion can also induce electrochemical dendrites resulting in leakage currents, which increasingly affect board reliability and life. Surface contaminations, which can cause parasitic capacities, distort the signal slew rate and often result in signal integrity disturbance and even product failure. For example, as systems in automobiles become more integrated, interactions between different components become more critical for the correct functioning of the system as a whole.

In summary, for any manufacturer producing mission-critical assemblies whose product performance envelope dictates a long product life, cleaning is a step that cannot be missed. Reliability requires cleaning.

Harald Wack, Ph.D., is president of Zestron (www.zestron.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Thursday, 31 March 2011 16:51
 

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