Authored By:
Beth Turner
ELECTROLUBE a division of MacDermid Alpha Electronics Solutions
Leicestershire, United Kingdom
Summary
Lightweight materials continue to add value to engineering, design, and technology, particularly as more organization’s vocalize their pledge to a sustainable future. The ability to do more with less affords numerous competitive advantages, lighter materials minimize waste through material saving, reduce consumption of finite resources, reduce environmental footprint, and achieve cost saving. Automotive design engineers reap considerable benefits when the weight of electronic components is reduced, lowering fuel consumption, reducing emissions, and improving the range of vehicles.
Traditionally, encapsulation resins are used as part of circuit board assembly to meet the performance challenges in a variety of electronic applications, such protective polymer materials are deemed essential to ruggedize electronic devices and give long lasting protection from harsh external environments and reduce negative impacts from long term wear-and-tear such as shock and vibration. Encapsulation resins are typically thermoset materials derived from crude oil and contain a significant amount of inorganic filler and thus have a high density and contribute significantly to the weight of the overall encapsulated device. This paper presents a novel, bio-based, expandable, thermoset solution, a low density, light weight method to effectively encapsulate and protect electronic components.
Numerous accelerated life tests were conducted to evaluate the performance of such light weight, expandable solutions alongside standard synthetic grades. Bare and reflowed TB33A test coupons were coated, and the Surface Insulation Resistance was recorded at 85°C (+185°F), 85% Relative Humidity, to understand the impact high temperature high humidity has on electrical resistivity. Test assemblies with a variety of surface mount components were subject to short term Condensation Testing, the Insulation Resistance was recorded.
A novel Foam Topcoat ruggedization solution was evaluated to highlight the value proposition of a hybrid ruggedization method that achieves considerable weight and material savings but still achieves a perfect score in a liquid water immersion test, equivalent to a high-density general-purpose encapsulation resin. Potted polycarbonate junction boxes were immersed in deleterious substances, to evaluate long term chemical resistance. This comparative study concludes that the proposed bio-based, expandable polymer acts as a lightweight solution to effectively protect an electronic device subject to harsh operating conditions, including high temperature high humidity, underwater and temperature cycles, and contributes to a significant weight reduction, greater than 80% weight saving is obtained when compared to traditional encapsulation resins.
Conclusions
Humid conditions drive corrosion products, monolayers of water can form on the surface of a PCBA and act as a carrier solution for any ionic contaminants that could be a residual artefact from any surface mount assembly such as paste reflow, or from environmental conditions such as salt-spray or corrosive gases, such factors along with dissimilar metals and bias cause corrosion products, any detrimental reaction is accelerated as the temperature increases. All polymers are permeable to water in its vapor phase.
Water vapor permeability reduces the insulation resistance of a ruggedized assembly. The impact of paste contamination is observed in this study, all ruggedization materials considered have a SIR one decade lower. Most general-purpose encapsulation resins contain inorganic powders that can be sensitive to hydrolysis in hot humid conditions, and such typically have poor performance in SIR testing. The bio-based expandable foam shows far superior protection and is considered hydrolytically stable on a Bare-PCB across a 1000-hour test period. This shows that we can provide more protection against hot humid conditions with 80% less material when comparing the SIR of the Foam with the Encapsulation Resin.
Liquid water, condensation brings about many of the same issues as corrosion, in addition liquid water can bridge conductors and lead to electrical shorting. Solvent based coatings perform poorly when exposed to short periods of condensation, the thin layers of non-crosslinking thermoplastic polymer have a lower resistance towards water absorption. Poor edge coverage on surface mount components can leave devices vulnerable to corrosion products. The Foam provides sufficient protection against condensation and is considered hydrolytically stable in this condensation test method. Expandable materials do a good job at protecting the intricacies of the complicated geometries of different surface mount components.
The Encapsulation Resin achieved a near perfect score in the condensation test but performed poorly in the SIR test under hot humid conditions. The Foam performed much better in the SIR test, it is considered hydrolytically stable in the conditions of the condensation test and shows far superior performance to the Solvent Based Coating when faced with different levels of short cycles of surface contamination of liquid water.
By considering the material consumption and weight of ruggedization material used to completely pot different volumes, it is potentially possible to get the ‘best of both worlds’ if the bulk of the volume is potted with a Foam and an impervious Topcoat layer applied to seal a unit, a significant 80% weight saving could be achieved, such monumental savings could be game changing to the overall economics of performance of ruggedization materials.
When monitoring the weight change attributed to water absorption of units potted with Foam and different thickness layers of Topcoat shows that a 500μm (19.7mil) layer of Topcoat material provides a sufficient barrier towards moisture and shows equivalent water absorption to a unit completely potted with a high-density general-purpose resin clearly displaying that the same barrier protection properties can be achieved with 80% less material.
Initially Published in the SMTA Proceedings
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