Back-to-Basic: Why do electronic assemblies need to be cleaned?
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Reliability is our driver - your cleaning solutions for SMT assemblies, power modules, stencils and maintenance.
Let’s be honest, if the original equipment manufacturer (OEM) or contract electronic manufacturer (CEM) of the electronics industry does not have to clean their assemblies, they wouldn’t. Nobody wants to invest the additional capital or commit operational funds and manpower unless it is absolutely necessary.
So, why has cleaning become such an important part of many electronics production lines?
Effectively removing contaminations is one step toward guaranteeing the long-term reliability of mission-critical circuit assemblies. While circuit boards in ordinary consumer products are not intended to last, high-reliability applications in the military, aerospace, communication, medical and automotive industries demand a guaranteed, long-term product life. This adds to the crucial need to remove several types and sources of contamination from circuit assemblies.
The possible impurities can be a result of:
Contamination is often found on the incoming bare boards and components. These include plastic, metal and fibreglass particulate residues from drilling and machining and salts from plating and etching operations. Furthermore, the process taken to ensure the long-term solderability of bare boards produces contamination that can compromise assembly performance. The commonly used method to ensure the extended shelf life of a bare board is hot-air solder leveling (HASL).
During this process, the primary sources of potential contamination include the ionic and polyglycol components found in the applied flux, as well as impurities such as oil and rust in the hot air directed onto the circuit boards. The fluxes exhibit a high level of aggressiveness, significantly elevating the board's ionic contamination level. The polyglycols in the flux are particularly troublesome as they can penetrate improperly cured coatings and leach out.
When components are manufactured, contamination may occur from metallization baths, rinse water, de-flashing chemicals, mould release agents and flux residues. Finally, during the assembly, issues may arise from the leftover solder paste, reflow condensates, outgassing, manual welding and handling residues. In particular, solder pastes occasionally produce small solder balls which can cause serious circuit reliability problems. In the assembly process, it is advisable to regularly inspect any new lot of bare boards and components for both, ionic and non-ionic contamination levels, arising from dust, oils, etc. This regular procedure helps minimize unexpected issues post-assembly.
Furthermore, if the cleaning agent, specifically formulated to eliminate soils produced during the assembly process, is not rinsed thoroughly, it can also introduce contamination. Even the water used in washing or rinsing procedures may leave behind residual impurities on boards if not adequately deionized.
So, how does contamination affect the reliability of circuit assemblies?
Circuit assemblies undergo cleaning processes to eliminate contaminants that could corrode joints and components, potentially leading to circuit failure. These failures can have several root causes such as:
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None of these phenomena are good for the reliability of the assembly as they will eventually cause it to fail. Circuit assemblies must operate and continue to function with exposure to various environmental conditions. Contaminants left on boards can compromise the circuit assembly by directly short-circuiting components and absorbing atmospheric moisture. As an effect, it reduces the resistance between component leads and promotes electrochemical migration and dendritic growth across connections.
In simpler terms, the combination of humidity and contamination may create conductive electrolytes. Applying stress voltage can then lead to electrochemical migration and dendritic growth.
Additionally, the sensitivity to contaminants rises when high-impedance circuitry is used. Therefore, while a small electric leak across a low-resistance component might not significantly impact a circuit's performance, the same leak could be disastrous across a high-resistance component. Currently, one of the latest trends in the industry is the growing prevalence of high-frequency (HF) circuits. Clean surfaces are essential for the proper functioning of this circuitry, and, naturally, the reliability of the end product is heavily dependent on the correct operation of these devices.
Conventional circuits with high-ohmic components are becoming more sensitive to climatic disturbances because minor current leakages can be interpreted as signals when they reach a similar current level. High-frequency circuits operating between 30MHz and 5 GHz exhibit even greater sensitivity to climatic disturbances.
To ensure the integrity of the signal, two crucial factors are required: high ohmic resistance and stable impedance. In the PCB design, it's essential to consider capacitive surface effects. Additionally, the environments where these high-frequency (HF) circuits operate are often characterized by high humidity and harmful gasses. This can cause failure due to the interaction between the flux residue and environmental contaminants if the assembly has not been properly cleaned.
In the present high-end assemblies, corrosion has the potential to trigger electrochemical dendrites, leading to leakage currents that progressively impact the reliability and lifespan of the boards. Surface contaminations, causing parasitic capacities, can distort the signal slew rate, often disrupting signal integrity and even product failure. For instance, in increasingly integrated automotive systems, the interactions between different components play a crucial role in ensuring the correct functioning of the entire system.
The RPM sensor on a wheel in an automobile supplies data not only to the ABS system but also to the engine management system. If the RPM sensor delivers inaccurate data, error traceability becomes difficult if not impossible.
In addition to the mentioned factors, issues can arise when boards are conformally coated due to residual contaminants. If contaminants are present on the surface, conformal coatings may not adhere properly. In conditions of suitable relative humidity, any water-soluble contaminant trapped beneath the coating can facilitate the passage of water vapour through the coating, leading to the dissolution of the contaminant.
The subsequent increase in volume can lead to blistering, causing the coating to separate from the surface. Electrochemical migration, dendrite growth, and electrical leakage currents pose inherent dangers.
In summary, for manufacturers producing mission-critical assemblies with a performance envelope demanding a prolonged product life, a thorough cleaning is essential as reliability hinges on proper cleaning procedures.
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