Ho-Kwang Yow

Investigation of mechanical shock testing of lead-free SAC solder joints in fine pitch BGA package

Abstract The lead-free Sn–Ag–Cu (SAC 305/405) solder that replaced the tin–lead eutectic solder tends to be more brittle in nature due to high stiffness and excessive solder interfacial reactions. This leads to higher occurrences of solder joints failure during surface mount assembly and handling operations as a result of PCB bending, shock impact and drop. In this work, mechanical tests simulating the shock impact were conducted on lead-free SAC of different weight percentages. These SAC materials were prepared for use in the solder joints of fine pitch ball grid array (BGA) components which were mounted onto the motherboard. After the mechanical shock tests, strain measurements were performed on the BGA components to gauge the solder joint integrity, which was shown to be related with the formation of intermetallics in the bulk and at the interface of the SAC solder. The ball pull tests were conducted to determine both the bulk and interfacial strength and the solder joint fracture, which was classified as either mode 1, 2 or 3. A correlation was made between the silver (Ag) and copper (Cu) weight percentages with the metallurgical reactions.

Analysis of the temperature dependence of current gain in heterojunction bipolar transistors

An analytical description is developed which highlights the important physical parameters influencing the temperature dependence of the current gain in heterojunction bipolar transistors (HBTs). Each of the possible base current components is discussed and its relative importance to temperature dependence is assessed. The manifold nature of the contributions explains the widely different conditions under which negative differential resistance has been previously reported. Space charge region recombination is found to be the main contribution to temperature variation of current gain at low current densities. Factors affecting modern highly doped base devices at high current densities and elevated temperatures are reverse hole injection and base bulk recombination.

High-temperature DC characteristics of Al/sub x/Ga/sub 0.52-x/In/sub 0.48/P/GaAs heterojunction bipolar transistors grown by metal organic vapor phase epitaxy

A series of Al/sub x/Ga/sub 0.52-x/In/sub 0.48/P/GaAs heterojunction bipolar transistors (HBTs) with x=0 to x=0.52 showed ideality factors close to unity for both base current and collector current and small variation in gain with temperature up to at least T=623 K across the whole range of x composition. Hole current injection from the base into the emitter in these devices was shown to be negligible. The current gain, /spl beta/, which is temperature insensitive was thought to be limited by bulk base recombination for x/spl les/0.3 and recombination at the graded emitter region for x>0.3. The optimum emitter composition (highest /spl beta/, and good /spl beta/ stability with collector current and temperature) was found to be x=0.18-0.30. Useful transistor action with very high gain and output resistance is possible up to at least T=623 K, limited only by the thermal performance of the unoptimized ohmic contacts employed in the devices.

Heterojunction bipolar transistors in AlGaInP/GaAs grown by metalorganic vapor phase epitaxy

A series of low‐pressure metalorganic vapor phase epitaxy grown AlxGa0.52−xIn0.48P/GaAs single heterojunction bipolar transistors with x=0, 0.18, 0.30, 0.40, and 0.52 have been studied. These devices consistently exhibit near‐ideal characteristics with base and collector current ideality factors close to unity except in the ungraded AlInP case. A low conduction band spike height which is believed to be due to unintentional grading at the emitter‐base junction of the partially graded devices (intentionally graded down close to the Ga0.52In0.48P lattice‐matched composition) provides effective hole current suppression over the whole range of compositions. Base bulk recombination current was determined to be the current‐gain limiting mechanism for x≤0.3 in this heterojunction bipolar transistors (HBT) system. The common‐emitter dc current gain showed a maximum value of 380 at JC=4×103 A/cm2 for x=0.18 which translates to a minority electron diffusion length of 1.1 μm. These are the highest values reported for...

GaInP/AlGaAs/GaInP double heterojunction bipolar transistors with zero conduction band spike at the collector

Al/sub 0.11/Ga/sub 0.89/As was used in the base, next to the GaInP collector of a DHBT, to eliminate the conduction band spike. The DHBTs demonstrated high breakdown voltages, BV/sub CEO/ and BV/sub CBO/ of 44.5 V and 54.5 V (gain/spl ap/20), respectively, for a 1-/spl mu/m-thick collector doped to 2/spl times/10/sup 16/ cm/sup -3/ with no voltage dependence of the current gain. Magneto-transport measurements were made on the AlGaAs bases and indicated limitations on the maximum practical base doping due to the inferior minority electron mobility and lifetime when compared with equivalently doped GaAs. Grading in the base from Al/sub 0.11/Ga/sub 0.89/As at the collector to Al/sub 0.21/Ga/sub 0.79/As at the emitter introduced a quasielectric field in the base, reduced the base transit time by a factor of /spl sim/2.5, and improved the gain over ungraded devices with the same average Al concentration.

Conduction band discontinuities in Ga0.5In0.5P‐AlxGa0.5−xIn0.5P heterojunctions measured by internal photoemission

The conduction band discontinuities in Ga0.5In0.5P‐AlxGa0.5−xIn0.5P heterojunctions, lattice matched to GaAs and grown by molecular beam epitaxy, were measured by internal photoemission techniques at room temperature over the whole compositional range. The discontinuity is found to vary linearly in x as (0.59x)eV for x≤0.30 and as (−0.18x+0.23)eV for x≳0.30, whereas the inferred valence band discontinuity (band‐gap difference minus the conduction band discontinuity) varies as (0.61x)eV. The direct–indirect gap crossover composition in AlxGa0.5−xIn0.5P is found to be close to x≊0.3.

Theoretical analysis of breakdown probabilities and jitter in single-photon avalanche diodes

A simple random ionization path length model is used to investigate the breakdown probabilities and jitter in single photon avalanche diodes (SPADs) with submicron multiplication widths. The simulation results show that increasing the multiplication width may not necessarily increase the breakdown probability relative to the breakdown voltage, as the effect of dead space becomes more dominant in thinner multiplication regions at realistic ionization threshold energies for GaAs. On the other hand, reducing the multiplication width results in smaller breakdown time and jitter, despite the increased dead space. The effect of dead space in degrading breakdown time and jitter is relatively weak and further compensated by the stronger influence of large feedback ionization at high fields. Thus, SPAD designs that can minimize the dark count rate may potentially benefit from enhanced breakdown probability, breakdown time, and jitter by reducing the thickness of the multiplication region.

GaInP∕GaAs double heterojunction bipolar transistor with GaAs∕Al0.11Ga0.89As∕GaInP composite collector

GaInP∕GaAs∕GaInP double heterojunction bipolar transistor (DHBT) with an Al0.11Ga0.89As layer within lowly doped GaAs–GaInP composite collector was characterized. In comparison to an abrupt GaInP∕GaAs∕GaInP DHBT with saturation voltages in excess of 20V, current gains of 25 at high biases, and breakdown voltages in the range of 22V, the DHBT incorporating GaAs–Al0.11Ga0.89As–GaInP composite collector has demonstrated lower saturation voltages of less than 6V and high current gains of 50 without compromising the breakdown voltages of the GaInP collector. Al0.11Ga0.89As layer can thus provide an alternative design to effectively minimize the potential spike effects at the GaAs∕GaInP heterojunction.

Analysis of Atomic Force Microscopy Images of Crystal Originated “Particles” on Silicon Wafers Treated with NH4OH:H2O2:H2O Solution

Crystal-originated particle (COP) side-wall angles and rates of change in width were measured after treatment in an SC-1 solution by atomic force microscopy (AFM) to determine the shape, size and type of the particles on a polished (100) Si wafer surface. The etched silicon tips maximum measurable slope angles were used to determine whether a COP originated from either the upper or the lower portion of an octahedral void. If the COP side-wall angle is equal to the maximum measurable slope angle of around 72°, the COP originated from the upper portion of the void. The lower portion of an octahedral void corresponds to a side-wall angle of 55°. This was confirmed through the study of COP width changes in NH4OH:H2O2:H2O (SC-1) solution. The COP width depends on the rates of change in width and the shape of its void. The rates of change of width of the COP which originated from the lower portion of the void was found to be 0.8 nm/min whereas that of the COP which originated from the upper portion of the void was 2.5 nm/min. It is also shown that a perceived single-type COP could very well be the upper part of a twin-type COP, which reveals its form of a much larger geometry only after the wafers are dipped repeatedly in the SC-1 solution.

Detection of Kite-Shaped COPs in Nitrogen-Doped Czochralski-Grown Silicon Wafers

Surface analyses by atomic force microscopy and field-emission scanning electron microscopy have revealed new shape of truncated voids, commonly known as crystal-originated-particles (COPs), that appear as kite-shaped pits with two (111) planes and two non-(111) planes in twin and triplet clusters, when viewed from the surface plane of nitrogen-doped Czochralski (CZ)-grown silicon (100) wafers. Also, the detection of higher number density but smaller size for these COPs indicates that the presence of nitrogen during CZ-growth of silicon crystals has strong influence on the size and density of COPs