Reliability and Cleanliness
One of the main factors influencing reliability is the failure of electronics assemblies by thermal fatigue of the solder joint. This applies to existing ceramic components such as capacitors, resistors and inductors, as well as ball grid components and thin small outline packages. It is not known what is the optimum solder fillet shape and volume. Too much solder can reduce the joint's compliance but a thin joint will be weak. At present there are various rules applied to pad geometries but there is little scientific justification for these. In the study at NPL the amount of solder paste applied to the joint and the pad area are being varied. The work is complimented by computer modelling (FEA) which uses the experimental data to qualify the software routines. The microstructural changes that occur during thermal cycling are being measured and their effect assessed.
With the advent of no-clean fluxes and fine pitch devices there is an increasing requirement to have no free residuals that can cause a breakdown in surface resistance between conductors.
A technique which measures the resistance across a standard pattern can be used to evaluate the free residual charge carriers left on the circuit board. The technique is known as Surface Insulation Resistance (SIR). The testing is carried out at elevated temperatures and humidities to accelerate the test. NPL is working to establish the test conditions for current fluxes and circuit geometries using the latest generation of test equipment with high sensitivity and fast data acquisition rates.
The pictures show the formation of dendrites, typically made up from dissolution products of the anode. If these occur on PCB they cause short circuits and failure.
NPL is leading the development of this test method in a European project to develop a new standard which is representative of today's fluxes and circuit geometries.
For more information: Chris Hunt