RTV Silicone Adhesive Sealants aid automotive electronics
07 April 2017
Improving component system reliability, reduction of component weight, thermal dissipation and the simplification of module assembly are all key issues affecting Tier One supplier TRW in its development of leading edge automotive products.
An approach has been adopted by TRW in its design and production of engine management modules that control injection and peripheral functions (such as emissions). In these units a key requirement is the prevention/ingress of moisture into the system. The innovative usages of RTV silicones have provided many benefits to TRW in its product design and production environments.
In the beginning
Initially these modules were based around die cast aluminium housings using soft cut gaskets to provide a seal and screws as the method of final assembly. Whilst functional there was potential for unwanted screws to rattle around and they represented both additional products and additional assembly operations.
Benefiting from new plastic materials
The development of a plastic housing made from a PBT/polycarbonate material provided the opportunity to build the connector as an integral part of the housing and achieve cost savings through the removal of peripheral seal screws through the usage of RTV silicone adhesive sealant.
Benefits of using RTV silicone within the assembly process
• Removes the need for a pre-cut fixed gasket.
• A hermetic seal was produced around the perimeter flange of the housing.
• The need for screws was removed and assembly was eased through the use of heat stakes, which, combined with the adhesive properties of the RTV silicone, provided an additional mechanical fixing mechanism.
• The RTV silicone prevented the ingress of moisture and protected against vibration
With many key component elements to the project (plastic housing, electronics, PCB, connector assembly and the sealing cover) there was a need for multi disciplinary meetings to consider materials/production and assembly issues that would be faced as production levels increased. At this point, UK Company Techsil, which has knowledge of RTV silicone adhesive sealants began to work closely with TRW.
Production issues requiring consideration
• Initially at the lower production volumes, speed of curing the lid sealant had proved satisfactory, with higher production volumes it would be important to reduce the length of time from final assembly to dispatch. Reduction of the cure speed of the sealing medium was considered an important factor to reduce production times.
• Analysis of production units also indicated a need for improved heat conduction within the unit.
• The mating surfaces of the male and female components of the units had been designed based upon mechanical design considerations with the u groove being designed into the aluminium cover with the plastic housing clipping into the cover. From a production process this resulted in the housing groove being filled with sealant and then flipped over and onto the plastic housing.
• Issues on self-levelling characteristics of the sealing material meant that it couldn’t be used in existing ovens as slight angles would lead to the material moving away from required sealing areas.
Opportunities presented by sealant changes
Analysis into the feasibility of moving from an air cure to a heat cure system was conducted and through careful material selection of heat cure products it was possible to reduce the speed of cure by 70 percent. Selection of a specialist thermal compound would allow for an improvement to the thermal path to remove heat from board to the heat sink.
The importance of production validation (PV)
With critical electronic units where moisture ingress is unacceptable, the units are pressure tested to validate the integrity of the seal. When conducting a PV test it was found that under pressure the units were leaking. Investigation into the problems identified the following:
1. In certain cases only limited adhesion was achieved between the RTV silicone and the plastic (i.e. not getting a hermetic seal)
2. Originally it was thought that a convection oven would provide a suitable mechanism for the heat cure, though tests indicated that this method was not performing consistently. A trial was conducted using Infra-Red heating, unfortunately the infrared required substantial heat within the assembly to generate the cure with the potential detriment to the electronic components.
Working with TRW personnel on-site, Techsil established that the modules were not getting hot enough within the convection oven for the heat cure process to effectively work and cure the material. Techsil was able to establish that inconsistencies in performance over time were down to subtle variation of changes in the tooling used to make the plastic moulding; the use and poor cleaning of mould release agent from the component parts and a slight bowing of components through amended tolerances.
In a module that now included an addition cure thermal transfer compound and a condensation cure gasket there were initial concerns that using the two materials together may result in inhibition issues. Techsil was able to conduct extensive tests to confirm that the materials were compatible.
Benefits provided through using RTV silicones within electronics assembly
• Through the use of RTV silicone within the assembly process, TRW removed more than 20 screws from within the assembly;
• An integrity of build that has improved the IP ratings achieved from IP44 to IP69;
• Usage of RTV gels has enabled the module to withstand vibration factors of six times the original production unit;
• Utilisation of updated thermal compounds has improved the thermal dissipation of the unit to improve by a factor of two
• Large leaded components (capacitors) used within the assembly have been protected through the usage of easily dispensed RTV Silicone that provide an elastomeric support
• PCB’s have been fully protected through the innovative usage of conformal coating that allowed even the tallest of electronic components on the board to receive complete coverage.
The use of UV tracer elements within the conformal coating allowed easy visual inspection of the board.
RTV Silicone adhesive sealants can provide many benefits when used within electronic assemblies. As production volumes increase, opportunities for selecting alternative RTV’s must be taken into consideration as significant production and technical benefits may be available.
Contact Details and Archive...