Design and manufacturing considerations for electronic assemblies
Improving component system reliability, reduction of component weight and the simplification of module assembly are all key issues affecting Tier One supplier TRW in their development of leading edge automotive products. When these are combined with manufacturing issues including; the ability to significantly ramp up production volumes; achieving increased levels of automated manufacture and reducing the number of individual components within an assembly then many factors become important in the design processes of the units.
An evolutionary approach has been adopted by TRW in their 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 their 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 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 sealant.
RTV silicone used in the assembly process removes the need for a pre cut fixed gasket and 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 provided an additional mechanical fixing mechanism. The RTV silicone prevented the ingress of moisture and protected against vibration
Moving from batch to volume production
As with all successful products, over time demand increased at which point a new set of production challenges needed to be resolved. One of these was the need to speed up the curing time of the lid sealant to reduce the time from final assembly to despatch. A heat cure system was considered and trialled but was found to have the potential to damage the electronic components.
With four key component elements to the project, plastic housing, electronics, PCB and 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 specialist knowledge of RTV silicones began to work closely with TRW.
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 a 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.
Every experience is a learning experiences
- When considering RTV silicones within an electronic design its one thing to consider the performance of a product on a specification sheet and another to see it implemented into a production process where all the nuances of mechanical design and other production issues become factors
- With multi disciplinary internal teams, an external materials specialist can work closely with engineers and provide additional support and practical experience that can save considerable time in overcoming problems as production volumes increase
- In utilizing RTV silicones it is critical to consider cause and effect issues i.e. changes to tooling can create issues to how an RTV will perform
Benefits provided through using RTV 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 6 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.
Summary
RTV Silicones 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 .
About Techsil
Techsil are industrial suppliers of RTV silicones, adhesives, sealants, encapsulants, conformal coatings, tapes, casting and mould making materials to a wide range of manufacturing industries across Europe including Aerospace, Automotive, Medical, Electronics, Railway, Lighting and Rapid Prototyping.
Techsil’s success has led it to become the largest RTV silicone distribution partner in Europe for Momentive Performance Materials (formerly GE Bayer Silicone). Through good partnerships with our suppliers, we are able to offer a complete range of sealing and bonding solutions to include well-established product ranges and new, technologically advanced materials. They are authorised distributors for MG Chemicals, ThreeBond, BJB, ITW Devcon, Power Adhesives, FastFixers, ITW Stokvis Tapes, VELCRO®, Araldite®, ChemTrend, Panacol and Hönle and also have a growing range of Techsil's own brand products.
For more information contact Techsil on technical@techsil.co.uk or visit www.techsil.co.uk
