PCBA Cleaning & Conformal Coating Increase Reliability of Critical Systems
Electronic systems in commercial, automotive, and military sectors have increased immensely over the last two decades due to technological advancements. With this increased dependency on electronics, it is more crucial than ever to ensure the reliability of these systems in varying climatic conditions. Humidity, dust, corrosion, and solder flux residues are some factors that make electronics vulnerable to malfunctions and failures. To protect the printed circuit board assemblies (PCBAs), the use of conformal coatings has become a common and effective practice in the industry.
FAILURE MODES IN PCBAS
Contamination is the main cause of intermittent and permanent failures in PCBAs. A small degree of contamination can boost the corrosion process. Some major sources of contamination include:
1.Solder flux residues
2.Moisture
3.Dirt
The leftover flux residues after soldering are acidic in nature. They can draw moisture from the air, which accelerates the corrosion of component leads and PCB contacts. In PCBA cleaning, “No-clean” solder paste is defined as very low-residue solder paste left after soldering (hand, reflow, or wave soldering).
Hand solder flux introduces most residues, but wave soldering can as well due the lower temperature (170 degrees) which doesn’t properly decompose flux residue on the top side. However, no-clean fluxes don’t need to be cleaned in most cases due to their low residue.
The second failure source is humidity. A non-coated PCBA will absorb more moisture compared to its coated counterpart. Humidity and contamination in the presence of the solder flux residues may cause electrochemical reactions causing oxidation at anodic conductors. In the presence of an electric field (external DC voltage bias), metal ions formed due to oxidation may drift towards cathodic conductors and initiate a reduction reaction. The reduced metal in the form of precipitates will form conductive tree-like metal filaments called dendrites. With time, dendrites will grow towards an anodic conductor and may ultimately result in a short circuit. This mode of failure is termed electrochemical migration (ECM).
Another failure mode in PCBAs is the reduction in Surface Insulation Resistance (SIR). The primary reason for the reduction in SIR is the leakage current between the conductors caused due to dendrite formation. This leakage current for example can cause serious degradation in the battery life of battery-operated low-current circuits. An important point to remember here is that not all dendrites cause a decrease in SIR. This is because when a short circuit is created due to dendrites, part of the dendrite is burnt instantly due to its minute cross-section. Only dendrites with a considerable cross-sectional area can affect SIR.
WHY USE CONFORMAL COATINGS ON PCBAS?
Conformal coatings are films made of synthetic materials (resins) or polymers that are applied to the PCBAs to protect them from moisture, corrosion, dirt, and contamination. Attributed to the inborn characteristics of these polymer coatings such as physical shielding, electrical insulation, hydrophobicity, counteraction to permeation of moisture, and gases, the long-term reliability of the PCBAs can be greatly enhanced [3]. Five types of coatings used in electronics are acrylic, epoxy, urethane, silicone, and parylene.
In PCBAs, SIR is the Key Performance Indicator (KPI) to test the reliability of the circuit board. The Institute of Printed Circuits (IPC) defines SIR as the electrical resistance between an insulating material, a pair of contacts, conductors, or grounding devices under specified environmental and electrical conditions. Ideally, SIR for a PCBA should be infinity under varying conditions. Besides preventing ECM and corrosion, conformal coatings are effective in increasing SIR.
PERFORMANCE OF CONFORMAL COATINGS
A conformal coating is known to prevent PCBAs from all types of contamination and failure modes. The adhesion strength is known to be the KPI governing the effectiveness of conformal coatings [4]. Adhesion strength depends on the substrate’s cleanliness and the chemistry of the coating. The presence of flux residues decreases adhesion strength. Since conformal coatings are semi-permeable, the existence of no-flux residues under the coating causes water build-up where adhesion is not strong. Consequently, corrosion accelerates in that region, and the effect of the coating is nullified.
After the introduction of ‘no-clean’ flux technology, the left-over flux residues were considered benign. Consequently, the practice of cleaning PCBAs before coating has diminished. However, studies reveal that the left-over so-called benign residues may still be able to cause significant damage to the PCBA.
Silicone, because of its high dielectric strength, high thermal and oxidative properties, its shock resistance, flexibility, and ease of repair is chosen as the coating material. The test substrates include a plain laminate and a PCBA made from an FR4 material. The laminate is covered with an epoxy solder mask while the test PCBA has surface mount technology (SMT) components (resistors and capacitors) mounted on it. The flux system under consideration is a ‘no-clean’ wave soldering flux. To draw a comparison, three types of surfaces are considered: clean surface, surface with non-activated residue, and surface with activated flux residue.
After exposing the laminates to 98% relative humidity (RH), the laminate coating with activated flux residue shows the greatest peeling whereas the clean surface shows the least peeling. The greatest peeling in the case of activated flux residue is attributed to the heating of the activator component (carboxylic acid). Heating carboxylic acid converts it to anhydride which is hygroscopic with a greater tendency to absorb moisture. The laminates with flux residues show blisters when exposed to 98% RH. However, some blisters disappeared after the laminates were returned to ambient conditions.
In the case of PCBAs, both the coated and uncoated boards with flux residues indicate an increase in leakage current. This failure mechanism (in SMT capacitors) is depicted in Figure 4. However, in the coated boards, leakage current builds after some time of normal operation.
Non-activated flux residues cause severe corrosion. An interesting point to notice here is that the SMT resistors are not affected by the presence of flux residues. This can be attributed to the glass protection layer on their surface. Whereas in capacitors, it is a rough layer made of ceramic material. From the study in[5], it is clear that PCBA cleaned before coating shows no signs of corrosion and no leakage current build up, which proves the importance of cleaning PCBAs before coating.
LINK:https://www.techspray.com/pcba-cleaning-conformal-coating-increase-reliability-of-critical-systems
