104 IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 23, NO. 2, APRIL 2000 The Influence of Solder Fillet Geometry on the Occurrence of Corona Discharge During Operation Between 400 and 900 V in Partial Vacuum Maurizio Materassi, Barrie D. Dunn, and Lorenzo Capineri Abstract—A number of experiments have been made to study the effect that different solder joint designs have on the voltage needed to initiate the onset of corona discharge under low ambient pressures. Wires were joined to both printed circuit board termina- tions and to electrical feedthroughs. All were constructed from ma- terials known to be suitable for spacecraft. The main factor that de- termines whether solder joints will be prone to initiate corona dis- charges or electrical breakdown in a space environment is the ac- tual minimum distance between terminals. Large, rounded solder fillets are desirable, but those containing protrusions or contours around wire strands have little adverse effect. It is shown that it is not always necessary, as is usually required by workmanship specifications related to high voltage connections, for operators to rework solder joints in order to achieve protrusion-free fillets. It would appear from metallurgical analyses of solder joints that it is the tin-rich phase that is sputtered from the fillet’s surface during the phenomenon of corona discharge. Index Terms—Corona discharge, high voltage, printed wiring board, reliability, rework, soldering, solder joint, spacecraft, vacuum, workmanship. I. INTRODUCTION A SERIES of experiments have been designed to assess the effect that solder joint workmanship has on the ability of spacecraft interconnections to withstand partial discharges or corona. The soldering operators who are engaged in the assembly of spacecraft high voltage electrical systems are trained and certified to produce soldered joints that are covered by smooth fillets, free of discontinuities or severe changes in contour [1]. Power systems for spacecraft have increased from a few watts in the 1960’s to the several kilowatts demanded by present space-based activities. As the power levels increase, so the transmission voltages will have to be increased in order to decrease currents and so minimize the conductor’s weight and resistance losses. This study concentrates on the typical soldered joints found on printed circuit board assemblies and wire bundle terminations as used in electronic power supply units and other power distribution circuits that operate between 400–900 V. Great care is taken to ensure that spacecraft high voltage con- nections (as well as terrestrial high capital-value electronics that Manuscript received August 12, 1999; revised February 1, 2000. M. Materassi is with the Proel Tecnologie Division, LABEN S.p.A., Florence, Italy. B. D. Dunn is with the European Space Agency (ESA-ESTEC), Noordwijk 2200AG, The Netherlands. L. Capineri is with the University of Florence, Florence, Italy. Publisher Item Identifier S 1521-334X(00)04557-2. Fig. 1. Specimens for test. operate in a vacuum environment) are fabricated to specifica- tion requirements, such as given in references [2] and [3], that prescribe for solder fillets having no sharp points and no rough- ness. Workmanship standards are often included in the speci- fications as artists’ impressions drawn to depict these smooth joints. For a variety of reasons operators may have great diffi- culty in achieving such objectives. These can include: the local poor solderability of cut ends of stranded wire or component leads, the large size of certain terminal pins and posts, diffi- cult accessibility with a soldering iron, as well as the inability to “build up” molten solder around the joint because the ma- terials being joined have both an excellent solderability and a high thermal conductivity. Many hours have been spent in at- tempting to rework such soldered joints in order to cover all wire strands and avoid every protrusion. Rework is usually per- formed under the control of a Materials Review Board (MRB), it is referred to as the reprocessing of a nonconforming item, to make it conform to drawing, specification, or contract. Unfor- tunately such rework is expensive, it often degrades or damages the hardware, and may not be technically necessary from a re- liability point of view [1]. It is generally considered important to avoid sharp points on conductors around which a corona can form and later initiate spark discharge. This will occur when the conditions for electrical breakdown are met (i.e., according to Paschen’s Law, the breakdown potential of a gas between elec- trodes is expressed as a function of the product of distance be- tween electrodes and the pressure of the gas) [4]. The present study aims to shed light on whether protru- sion-free solder fillets are technically required for spacecraft 1521–334X/00$10.00 © 2000 IEEE