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60 SMT Magazine • June 2015 eVenT ReVIeW: (eMPS-6) continues voiding at the progressing Cu 3 Sn intermetallic interfaces. Further work will be conducted at Aalto University to understand the reliability of hermetically sealed MEMS devices, together with the measurement of accurate leak rates. The final session on the second day related to the failure modes or potential failure modes related to spacecraft electronics. Ronald Schonholz from Isola- Group in Germany reviewed the well-known failure mecha- nism caused by the growth of conductive anodic filaments (CAF) within multi-layer boards. This is particularly a problem for spacecraft elec- tronics due to their progres- sive miniaturization. Space- craft PCBs can be subjected to moisture during ground testing, and also when as- sembled boards are thermal cycled from cold to hot envi- ronments they can be exposed to condensed water. The electro- chemistry related to CAF for various laminates was discussed, in particular the weave of laminate glass fibre fabrics was deemed to be very important. New fabrics used by Isola have a more even distribution of glass filaments and are better wetted by resin during manufacture. These are so-called "spread fibres" with Silan finishes (providing a better Si-O-Si-coupling agent to polymer) result in a far better chemi- cally bonded interfaces between glass and resin. Examples of CAF failures were demonstrated by metallographic images for the old fibre systems. Following the CAF review, Poul Juul of the Hytek Institute, Aalborg, Denmark, demon- strated electro-chemical migration (ECM) be- tween tracks on PCBs. Examples of ECM caused by flux residues were shown. Many examples of laminate failures were illustrated; their causes were said to be often related to insufficient bonding between resin and glass fibre, hollow fibres, poor drilling and void formation. Juul emphasised the urgent need to retain highly skilled persons in the European PCB manufac- turing industry (process and chemical engi- neers, laboratory technicians and operators). Inventec, part of the French Dehon Group, was represented by J.H. Serzisko. This company produces hydrofluoroethers which are clear, co- lourless and low odour liquids, partially halo- genated carbon bonds, with low boiling points. This liquid material is used in specialised clean- ing tanks for aerospace mechanisms, electronic assemblies, spacecraft mechanisms and fire protection systems. This cleaning liquid meets all the legal requirements such as REACH and the cleaning equipment requires no local extraction. Continuing with the theme of cleaning for space- craft electronics, Graham Naisbitt, director of GEN3 Systems, Farnborough, UK, reviewed the topic of flux res- idues and the need to clean such contaminants prior to conformal coating. All fluxes leave residues which might compromise electronic assembly performance. It is essential to know the electrochemistry of such fluxes and it is strongly recommended that the surface insula- tion resistance (SIR) of any residues that may be present after cleaning are fully understood. SIR and ROSE testing are the recommended meth- ods for determining whether circuit reliability will be compromised by contaminants includ- ing the so-called "no-clean" flux residues. Other techniques such as ion chromatography may be effective at measuring the presence of ionic salts but the method is not suitable for assess- ing whether reliability will be compromised. SIR testing evaluates whether electrochemi- cal failures may occur in the presence of hu- midity, ionic contamination and bias. Various ionic contamination test methods are available (ROSE and SEC). These determine the amount of salt (equivalent) remaining on both bare and component-assembled printed circuit boards. The tests involve a 75/25% mix of IPA with de-ionised water. The conductivity of the mix solution is known, and once a contaminated test PCB assembly is agitated in the solution a second reading computes the dissolved ionic artICle sIr and rose testing are the recommended methods for determining whether circuit reliability will be compromised by contaminants including the so-called "no-clean" flux residues. " "