Why You Shouldn't Directly Apply Insulating Glue on Logic Boards from the Perspective of Electronic Components？

 Recently, a customer from South Korea, while inquiring about 15.6-inch LCD display kit, had a question regarding the application of insulating glue.

Drawing on over 20 years of production and design experience, our engineers provided the following response.：
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Logic boards, such as printed - circuit boards (PCBs), are the nerve centers of electronic devices, housing a complex

network of electronic components. While the idea of applying insulating glue directly onto a logic board might seem appealing for protection, it can lead to several critical issues from an electronic component perspective:

1. Electrical Performance Interference

Glue can seep into the tiny gaps between component leads or pads (usually less than 0.5mm). This intrusion can increase parasitic capacitance or reduce insulation resistance. In high - frequency circuits, even a slight change in these parameters can severely distort or attenuate signals. For example, in 5G communication modules operating at multi - gigahertz frequencies, a small increase in parasitic capacitance due to glue can lead to significant signal losses.

Some glues contain ionic impurities. Over time, under the influence of an electric field and humidity, these impurities can cause electrochemical migration (ECM). ECM can create conductive paths between adjacent components, ultimately resulting in short - circuit failures.

2. Thermal Expansion Stress Damage

The coefficient of thermal expansion (CTE) of common glues often does not match that of the FR4 substrate (about 17ppm/℃). During temperature variations, typically from - 40℃ to + 85℃ in industrial applications, the mismatched CTE generates stress at the component solder joints.

Components like ball - grid array (BGA) packages, which have very small solder balls (with a diameter of only 0.3mm), are particularly vulnerable. The stress can cause the solder balls to crack. According to the IPC - J - STD - 020 standard, the stress on solder joints should be less than 10MPa to ensure reliable operation.

3. Deterioration of Heat Dissipation Performance

Glues generally have a low thermal conductivity, usually less than 0.2W/(m·K), while that of air is 0.026W/(m·K). Although it might seem counterintuitive that a glue layer would worsen heat dissipation compared to air, in practice:

A 1 - mm - thick glue layer can increase the junction temperature of power devices by approximately 15℃, as measured based on the JEDEC JESD51 - 2 standard. In high - power applications such as server CPUs, this increase in temperature can trigger over - heat protection mechanisms, reducing the device's performance.

4. Loss of Maintainability

Epoxy - based glues, once cured, can have a shear strength of over 20MPa. When attempting to remove components for repair or replacement:

For quad - flat - package (QFP) components with a minimum lead pitch of 0.4mm, the force required to remove the component can cause lead deformation.

In the case of BGA packages, the solder pads, which are only 0.25mm in diameter, are likely to be ripped off.

Delicate sensors, such as MEMS gyroscopes, can be easily damaged during the disassembly process.

5. Material Compatibility Issues

Some glues with acidic curing agents, like amines, can corrode the cathode foil of aluminum electrolytic capacitors, which is less than 10μm thick. This corrosion can lead to capacitor failure, affecting the power - supply filtering and stability of the circuit.

UV - cured glues may yellow when exposed to long - term ultraviolet light. This discoloration can interfere with the operation of optoelectronic devices such as fiber - optic modules, as it can absorb or scatter light signals.

Professional Alternatives

It is recommended to use conformal coatings that comply with the IPC - CC - 830B standard, such as Parylene C. Parylene C, with a thickness of 1 - 5μm, offers:

High dielectric strength of over 400V/μm, ensuring excellent electrical insulation.

A CTE that closely matches that of common PCB materials (around 30ppm/℃), minimizing thermal stress.

A very low moisture permeability rate of less than 0.1g/m²·day, protecting components from humidity - related degradation.

It can also pass the UL746E certification, indicating its reliability and safety.

In special scenarios, nanocoating technologies, such as Siloxane - based coatings, can be considered. These coatings have a contact angle of over 110°, providing excellent hydrophobicity. They can achieve an IPX8 waterproof rating while still allowing for gas permeability, which is crucial for some applications.