Feature: Avionics
2. Contamination Contamination is another source of ESD. Various contaminants, including dust, dirt, oils, greases and flux residues, can accumulate on PCBs and electronic components during handling and soldering processes. Tese contaminants elevate the surface conductivity of PCBs and components, promoting static charging through friction between dissimilar materials. Additionally, they diminish surface resistivity, hindering the natural dissipation of charges. Contaminants on the PCB surface or
between closely spaced components can also affect electrical properties and signal integrity. For instance, dust particles have the potential to cause current leakage or signal interference, or they can lead to intermittent connections. Given these risks, thorough cleaning
is imperative in electronic design and manufacturing. By effectively removing particulates, residues and contaminants, cleaning minimises ESD occurrences, reduces signal degradation and improves
PCB performance and reliability. However, cleaning the PCBs within avionic systems presents a daunting task due to their intricate designs and densely-packed parts. Te ongoing trend toward greater complexity and miniaturisation worsens this challenge, making it increasingly difficult to reach and clean inaccessible areas. Consequently, the imperative lies in developing effective cleaning procedures capable of reliably removing contaminants without compromising delicate components.
ESD control
1. Manual cleaning Effective ESD control strategies involve environmental control measures like grounding workers and workstations, keeping suitable humidity levels, and avoiding the use of compressed air for drying PCBs. Routine surface cleaning with ESD-reducing wipes and the use of static-zapping dispensing tools and controlled flux-remover dispensing systems when manually cleaning PCBs
can further minimise ESD risks. Tese tools ensure thorough cleaning whilst reducing static charges, making PCBs reliable and long-lasting.
2. Vapour degreasing Vapour degreasing stands out as an exceptionally effective cleaning technique for cleaning larger batches of avionic PCBs. Tis method entails submerging the PCBs in a specialised cleaning fluid within a vapour degreaser machine; see Figure 3. Technicians immerse the PCBs in the fluid, which is continuously filtered and distilled, effectively dissolving and liſting contaminants from the surface. In some cases, ultrasonic agitation may be employed to augment cleaning efficacy. Following immersion, the PCBs undergo
a swiſt vapour rinse and drying. Te cleaning fluid condenses and returns to the vapour degreaser for reuse, a process that can be repeated hundreds of times before requiring replenishment. Tis not only streamlines operations but also minimises the need for hazardous waste disposal,
Figure 2: ESD can originate from ungrounded electrical testing equipment
38 April 2024
www.electronicsworld.co.uk
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