PCBA Cleaning Before Conformal Coating: Increase Reliability & Functional Life
In the electronic industry, the manufacture of printed circuit-board assemblies (PCBAs) has been greatly simplified by the adoption of “no-clean” fluxes during the soldering process. This “no-clean” solder paste intends to eliminate the need for cleaning the circuit boards after assembling all the metal parts into them. This type of flux is marketed to, and assumed to not leave significant amounts of active residue on the board’s surface after soldering, but multiple studies suggest that this is not necessarily the case.
No-clean” fluxes can save expensive steps in electronic card manufacturing, they are safe, effective, and reliable when used properly. However, it has been demonstrated that even “no-clean” solder paste can lead to electrochemical migration and dendritic growth, which are one of the most common causes of failures in PCBAs. With this in mind, this article aims to showcase the importance of cleaning even “no-clean” fluxes in printed circuit board assemblies.
WHY CLEAN ELECTRONIC ASSEMBLIES?
Cleaning PCBAs before applying the conformal coating is a critical step that influences the reliability and performance of electronic devices. Conformal coatings are thin polymeric films applied to PCBAs to safeguard them against environmental factors such as moisture, dust, chemicals, and corrosion. It enhances the circuit board's longevity and prevents product returns for manufacturers. However, failure to adequately clean the PCBAs before coating can lead to a range of problems that undermine these protective benefits.
Contaminants such as flux residues, dust, and other particulates left behind during the assembly process can impair the adhesion of conformal coatings to the PCB surface. This compromised adhesion can result in coating delamination, reduced effectiveness in protecting against moisture, and increased vulnerability to corrosion. Even in cases where “no-clean” solder pastes are used, the residues they leave behind can interfere with coating adhesion, and negatively impact the board’s performance.
Moreover, one of the most concerning consequences of insufficient cleaning is the formation of dendrites, which are microscopic conductive filaments that can bridge closely spaced conductive traces on the PCB. This might lead to short circuits, electrical failures, and compromised functionality, posing significant risks to the device's operation and potentially resulting in costly recalls or repairs. It is estimated that failures due to electrochemical migration have increased in recent years compared to what the industry experienced in the 70’s through 90’s.
HOW DO DENDRITES GROW ON ELECTRONIC ASSEMBLIES?
Flux residues on the circuit’s board surface can lead to a process called Electrochemical Migration. It is the formation of conductive metal filaments across a printed circuit board (PCB) when dense current leakages occur, in the presence of an electrolytic solution. The outcome of this process is the formation of dendrites, which cause surface contamination, device sensitivity, and a rise in flux residues. These dendrites are a common root cause of field failures, and they can be either intermediate or permanent.
PCBA's reliability is highly affected by the chemistry of the soldering materials, and the activity and ionic nature of the residues. Other key factors that can contribute to electrochemical migration are the PCB material composition (plating chemistry), uncleaned flux residue, active chemistry concentration and distribution, board surface roughness, and the environmental conditions to which the PCB gets exposed (e.g., Humidity, Temperature).
An overview of the electrochemical migration process inside a PCBA is presented in Figure 1. The chemical reaction within a PCBA involves the copper in the board as well as the inorganic and organic compounds that constitute the substrate. Other substances, such as the flux, can also participate in the electrochemical migration process, which is divided into three (3) steps:
1. Metal dissolution: A strong alkaline condition attacks the anode and the power-leading contacts respectively – usually created by environmental pollutants or flux residue. The reaction spreads from the deplating anode to the cathode, forming dried corrosive residue along the printed circuit board.
2. Ion Migration: It is determined by the concentration of the ions, in which they spread out to the areas where there is an alkaline condition. This step is the precursor for the dendrites to grow along the concentration gradient. The current flow promotes the upcoming dendrite formation, and potential short on PCBAs.
3. Dendrite Growth: Near the cathode (ground potential), there is normally a high acidic condition and a positively charged ion concentration. The reduction in negatively charged ions leads to the deposition of positively charged ions as metal dendrites. Dendrite growth is just a micro plating cell that drives the metal to build a bridge between the cathode and the anode.
The whole process finally results in highly conductive bridges or filaments, that build up along glass fibers in the PCB substrate material. The composition of these formations can vary from pure metal (slow growth) to metal-metal oxide composites (moderate growth).
LINK：https://www.techspray.com/pcba-cleaning-before-conformal-coating-increase-reliability-functional-life
