Harold F. Webster

Solder Fatigue Problems in Power Packages

The fatigue of solder connections in power packaging has been investigated. Power devices use large bond areas which lead to fatigue life problems. Three different types of structures consisting of silicon diodes attached to copper heatsinks have been thermally cycled and the integrity of the bonds compared. The thermal resistance from the silicon diode to its heatsink has been used to measure bond quality. Use of n BeO strain buffer between the silicon chip and the copper heatsink (Type II) improved the solder cycle life. Further addition of a layer of structured copper between the BeO and the copper sink (Type III resulted in marked additional improvement in cycle life. Fatigue failure was indicated by a rapid rise in the thermal resistance that suggested a crack or tear in one of the bonds between the silicon and the heat sink. This model has been confirmed by an ultrasonic microscope scan. Solder fatigue life was also extended by the use of hard solders, compressive forces, and hermeticity.

Characterization of via connections in silicon circuit boards

Conducting vias, isolated by silicon dioxide from a bulk silicon wafer and used to interconnect stripline transmission lines on opposite surfaces of the wafer, are analyzed. The net VSWR (virtual standing wave ratio) and insertion loss for a single via and the crosstalk or coupling between two nearby vias are determined as a function of geometry, frequency, and silicon resistivity. For small via dimensions and frequencies up to 1 GHz, the analysis predicts low VSWR and low insertion loss, provided the silicon resistivity is greater than about 100 Omega -cm. It is shown that crosstalk can be small, and is mostly due to inductive coupling. >

Structure in Magnetically Confined Electron Beams

A number of observations have been made of structure changes that occur in hollow and solid electron beams which are confined by a magnetic field. These structure changes occur in both the density of the beam and the transverse velocity components of the beam electrons. The velocity components have been measured by use of a pinhole camera.Some of the results obtained are as follows. The density of electron paths in a beam may become non‐uniform if there is a spread in the forward component of velocity of the electrons. This may occur either because of a spread in initial angle of the electrons or because of a potential depression in the beam.In addition, changes in beam shape and transverse velocity components of the beam electrons can occur as a result of drifting of electrons in crossed electric and magnetic fields, the electric fields in this case being provided by the space charge of the electrons themselves. Thin beams have been found to be unstable and it has been observed that they divide into an a...

Calculation of the Performance of a High‐Vacuum Thermionic Energy Converter

The performance of a high‐vacuum thermionic energy converter has been evaluated from Langmuirs 1923 paper on the thermionic diode. The results are presented in the form of a generalized set of curves which show output voltage as a function of current drawn from the device. These general curves have then been applied to a few specific cases to determine what cathode‐anode spacings, and cathode and anode properties will be required to produce a practical energy converter.

Thermionic Emission from a Tantalum Crystal in Cesium or Rubidium Vapor

Thermionic emission was measured from a hemispherical tantalum crystal with its work function altered by alkali metal sorption. Data are graphically presented. Emission density is strongly dependent upon the crystallographic face of tantalum, and emission pattern changes are different for cesium and rubidium.

Structured Copper: A Pliable High Conductance Material for Bonding to Silicon Power Devices

The large difference in thermal expansion between silicon and the high conductivity metals is a major problem to be solved in the packaging of high power silicon devices. One solution is by the use of structured copper which is made of many separate copper wires and can he directly attached to silicon without introducing large stresses. Methods of preparing structured copper are presented along with some examples of its application.

Liquid and Solid Cesium on a Rhenium Substrate

Cesium metal has been deposited from the vapor on a large‐grained rhenium substrate. Both the wetting of the substrate by the cesium and the mode of growth of solid films of cesium are dependent upon the crystal face exposed by the various grains of rhenium. The experiment was done in a sealed‐off glass tube which had a residual gas pressure of 10−9 Torr before cesium was admitted. Both the wetting and the mode of film growth were dependent upon monolayer amounts of impurities. Epitaxial relationships were observed to exist between many of the rhenium grains and the deposited cesium.