PCBs
Why cleaning is critical for aerospace PCB reliability
M
odern aircraft uses sophisticated avionics to control a variety of critical functions. These electronic systems include everything
from communications and engine controls, to flight recorders, security systems and radio communication systems all using complex printed circuit boards (PCBs). These devices must withstand exceptionally challenging conditions over long periods of time. Extreme temperatures, high humidity and excessive vibration are just some of the demands that the delicate circuitry must endure. Furthermore, PCBs in aerospace applications are often used in areas where replacing the assembly can be difficult or nearly impossible. It is therefore crucial that the PCBs function reliably and without fault.
Contaminant risk and the challenges of cleaning
One of the main reasons for PCB failure is contamination. The smallest contaminant can impair PCB performance, so precision cleaning is critical to reliability. Aerospace circuit boards are vulnerable to a variety of different contaminants or soils during the production process. Whether it is through the manufacturing stage, or during transport and storage, the chance of contamination is high. Possible contaminants include everything from
boiling cleaning fluid to loosen and dissolve most of the contamination. They then move to the rinse stage where any final trace of contamination is completely removed. Finally, the PCBs are held inside the vapour blanket. As the PCBs moved through the vapour blanket into the freeboard chiller zone, they dry quickly and come out of the vapour degreaser dry and cool. This is especially important since the PCBs must be cool to allow conformal coatings to adhere to the boards properly.
dust, adhesives and oil, to uncured solder paste, flux activators and water. All contaminants must be removed as soiled PCBs run a high risk of malfunction or even complete failure. This however, can be a challenge as aerospace PCBs have become smaller to save space and reduce weight.
PCBs used within avionics are becoming increasingly multifaceted with double-sided and multilayered layouts the norm. Designers are adding more micro components like CSP, flip chip, micro BGA and QFN packages into tighter and smaller spaces on the PCBs. Low standoff components like MOSFETs and zero-clearance parts are now routine.
The challenge is that as PCBs decrease in size and grow in density, successful critical cleaning is more difficult to achieve. Dirty PCBs
are susceptible to a whole host of problems. Electrochemical migration, delamination, parasitic leakage, dendrite growth and shorting are just some of the consequences. It is, therefore, crucial to remove all contaminants from PCBs used within the aerospace sector.
Vapour degreasing
Vapour degreasing is an effective method to successfully clean the complex miniaturised PCBs used within avionics. When combined with modern, sustainable, solvent-based cleaning fluid, it ensures boards are precisely cleaned. Vapour degreasing uses cleaning fluid immersion, followed by vapour rinsing and drying, to clean PCBs down to the submicron range. It removes all types of contaminants including fluxes, pastes, oils and particulate. The vapour degreaser cleaning fluid has a low surface tension. This allows the fluid to permeate the entire board, including under and in-between tightly-spaced and low-mounted components. The cleaning fluid flows under components, dissolves contaminants, and most importantly washes them completely away without leaving residue. Its low boiling temperature minimises the risk of damage to delicate PCB components. In addition, vapour degreasers recycle and reuse the cleaning fluid for hundreds of hours before the fluid is refreshed, making it a cost-effective and environmentally sound cleaning method. The vapour degreaser applies heat to the cleaning fluid to produce a low temperature boil. This produces a pure, clear and dense vapour blanket. PCBs are lowered into the
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Cleaner boards enhance coating Any contaminants on PCBs can inhibit bonding of the conformal coating to the PCB substrates. For example, if salts or oils from fingerprints are left on the boards, they can cause coating defects like uneven coverage, pinholes and craters.
In addition, flux residue absorbs and traps moisture which releases during coating curing operations. This causes separation of the coating from the board, allowing corrosive materials, dust, or water from the end-use environment to penetrate the PCB assembly. These harmful elements can cause corrosion, signal loss, or even complete component failure. To prevent performance problems, residue-free cleaning prior to conformal coating is critical.
Clean correctly to avoid failure Removing any harmful contaminants and residue from aerospace PCBs is vital to its uninterrupted service life. If not cleaned effectively, the PCB simply will not function reliably for the lifespan of the board, resulting in expensive repairs or replacements. Vapour degreaser cleaning helps to ensure the cleanliness of the PCB. It is also a fast and efficient process that increases cycle time and production yield.
When it comes to aerospace applications, PCB cleanliness is not just important it is crucial. For the best PCB cleaning success, partner with a cleaning fluid supplier with both PCB cleaning and vapour degreasing expertise to provide the best cleaning advice.
microcare.com Components in Electronics March 2021 37
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