Improving PCB reliability with cleaning

AMJuly20Features - microcare1
AMJuly20Features - microcare1

Emily Peck, a senior chemist at MicroCare, examines the importance of quality cleaning of PCBs to ensure they work effectively and reliably.

 

Printed circuit boards (PCBs) are used in a variety of aerospace equipment including aircraft, satellites and radio communications systems. Many are exposed to harsh conditions including extreme temperatures, high humidity and excessive vibration. They must endure life-cycles that often last for decades.

Some aerospace applications include situations where replacing a circuit board can be difficult or nearly impossible. For example, accessing a PCB on a GPS satellite or deep inside an aircraft navigation system takes an enormous amount of time, effort and expense. Therefore, it is crucial that the PCBs function reliably and without fail.

Critically clean for reliability

Circuit board manufacturers are careful to produce the best boards for critical aerospace applications. This includes exacting designs, careful materials selection and fastidious production methods. But for overall reliability, one particular production process requires special attention - PCB cleaning. For aerospace PCBs to perform their critical functions as intended, they must be critically clean.

Production contaminants

Aerospace circuit boards are vulnerable to a variety of different contaminants or soils during the production process. Contaminants come from many sources including transport, handling, storage and manufacturing. Soils include everything from dust, adhesives and oil from fingerprints, to uncured solder paste, flux activators and water. All contaminants must be removed, otherwise soiled PCBs run a high risk of malfunction or even complete failure.

The challenges of clean

Aerospace PCBs are getting smaller to save space and reduce weight. They are also becoming more complex with double-sided and multilayered layouts. Designers put 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.

PCBs cleaned in a vapour degreaser come out clean, dry and cool

The challenge is that as PCBs shrink in size and grow in density, cleaning them becomes more critical and more difficult. Dirty PCBs are vulnerable to a variety of problems including parasitic leakage, electrochemical migration, delamination, shorting and dendrite growth. So, it is imperative to remove all contaminants from these tiny, tightly-stacked, high-density and complex boards.

Vapor degreaser cleaning

A good solution for effectively cleaning small, complex PCBs is to use a vapour degreaser with a modern, sustainable, solvent-based cleaning fluid. Vapor degreasing uses cleaning fluid immersion, combined with vapour rinsing and vapour drying, to clean PCBs down to the submicron range. It removes all types of residue including fluxes, pastes, oils and particulate.

The cleaning fluid has a low surface tension allowing it to permeate the entire board, including under and in-between tightly-spaced and low-mounted components. Its low boiling temperature also minimises the risk of damage to delicate PCB components. And since the cleaning fluid is ultra-pure, it leaves no residue behind.

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.

How it works

The vapour degreaser applies heat to the cleaning fluid to produce a low temperature boil, usually between 40˚C/105˚F and 65°C/165°F. This produces a pure, clear and dense vapour blanket. PCBs lower into the boiling cleaning fluid to heat, loosen and dissolve the majority of the contamination. They then move to the rinse stage where any final vestige of contamination washes away.

Maximise takt time

Finally, the PCBs raise up and hold inside the vapour blanket, allowing the parts to dry and cool. The entire process is extremely efficient and helps maximise takt time. It can take as little as 6-20 minutes per batch, and is customisable to the cleaning result needed.

Ready for conformal coating

The vapour blanket dries the PCBs quickly. They 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.

Cleaner boards, better coating

Any contaminants or soils on PCBs may interfere with the proper 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 defects in the conformal coating. These include uneven coverage, pinholes, craters, blisters and fish eyes.

As PCBs get smaller, they are more difficult to clean

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, cleaning the contaminants off the PCB prior to conformal coating is absolutely essential.

To sum up

Removing any harmful contaminants and residue from an aerospace PCB is key to its uninterrupted service life. If not cleaned effectively, the PCB simply will not function reliably for the required life of the board, causing timely and expensive repairs or replacements. Vapor degreaser cleaning helps ensure the cleanliness of the PCB. It is also a fast and efficient process that increases takt time, production yield and PCB reliability.

The vapour degreaser cleaning fluid should be strong enough to remove the contaminant, yet not damage the substrate including the composite or hybrid materials that are used more often in today’s PCB designs.

Removing harmful residue from a PCB is key to its reliability

To determine the best cleaning fluid to use, work with a knowledgeable cleaning partner that specialises in aerospace PCB cleaning. These experts help PCB producers choose the best fluids to deliver quality cleaning results and ensure the aerospace PCB’s reliability and longevity.

Emily Peck is a senior chemist at MicroCare, which offers benchtop and vapour degreasing electronics cleaning solutions. She has been in the industry more than six years and holds a MS in Chemistry from Tufts University. Peck researches, develops and tests cleaning-related products that are used on a daily basis in electronics, medical and precision cleaning applications.

www.microcare.com

Company

MicroCare

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