Howard Douglas Blair

Intermetallic compound growth on Ni, Au/Ni, and Pd/Ni substrates with Sn/Pb, Sn/Ag, and Sn solders [PWBs]

The growth mechanism of the Ni/sub 3/Sn/sub 4/ intermetallic compound (IMC) during aging was studied with three different solders (100Sn, Sn-3.5Ag, and Sn-37Pb) on three different substrates (Ni, Ni/Au, and Ni/Pd), at the temperatures of 75, 100, 125, and 160/spl deg/C from 1 to 36 days. The growth rates of Ni/sub 3/Sn/sub 4/ with Sn on Ni and Ni/Au substrates were similar, growing to about 6 /spl mu/m after 36 days at 160/spl deg/C, but only to about 1-2 /spl mu/m after 36 days at a temperature below 100/spl deg/C. The growth rate of Ni/sub 3/Sn/sub 4/ with Sn-37Pb on Ni/Au substrate was close to that with Sn for the same substrates. However, the Sn-3.5Ag solder showed a slower growth rate of Ni/sub 3/Sn/sub 4/ on both Ni and Ni/Au substrates, resulting in only about half the thicknesses when compared to Sn on the same substrates. In addition to the Ni/sub 3/Sn/sub 4/ compound, a PdSn/sub 4/ compound was observed on the NiPd substrates. The growth rate of Ni/sub 3/Sn/sub 4/ on the Ni/Pd substrate is much slower than that on either the Ni or the Ni/Au substrate, possibly due to the existence of the PdSn/sub 4/ layer between Ni and the solder. At temperatures lower than 100/spl deg/C, there is hardly any Ni/sub 3/Sn/sub 4/ detected for Sn-3.5Ag and Sn-37Pb solders for up to 36 days. The apparent activation energies, Q, are in the range of 3 to 12.8 Kcal/mole, and Q for Ni/sub 3/Sn/sub 4/ with Sn is the highest for the three solders on both the Ni and Ni/Pd substrates, and those for Sn-3.5Ag the lowest. However, Q for Ni/sub 3/Sn/sub 4/ growth with Sn-3.5Ag is the highest on the Ni/Au substrate. A thick Ni/sub 3/Sn/sub 4/ layer may pose potential reliability issues as evidenced by the fractured morphology of the intermetallics due to a 10.7% volume shrinkage during the transformation from solid phase Sn and Ni to the Ni/sub 3/Sn/sub 4/ compound.

Current carrying capacity of copper conductors in printed wiring boards

The effect of thermal management of the copper conductors on PWBs (printed wiring boards) having different dimensions and arrangements is discussed. Design charts have been generated to include the parameters of conductor thickness from 1 to 3 oz, width from 5 to 20 mils, spacing at 8 mils, board thickness from 31 to 62 mils, input current, and temperature rise up to 50/spl deg/C. The analysis is based on finite element modeling with a heat transfer film coefficient obtained from infrared thermal imaging analysis of a test board. Of all the geometric parameters considered, conductor width and spacing are the primary parameters influencing thermal resistance. Conductor thickness is next, and board thickness proves to be the least sensitive parameter.<<ETX>>

Experimental Analysis of Thermal Cycling Fatigue of Four-Layered FR4 Printed Wiring Boards

Long-term reliability of electronic packaging has become a greater challenge as a result of ever increasing power requirements and the decreasing size of electronic packages. In this study, the effects of three variables on plated-through hole (PTH) design have been investigated on the thermal cycling fatigue lives in four-layered printed wiring boards (PWB’s). These three variables were evaluated at two levels each: (a) hole size (0.030 and 0.040 in.), (b) internal pad (presence or absence), and (c) epoxy-plugged holes (plugged or unplugged). The electrical resistance was measured on 40 test boards with 23 design of 8 daisy-chain PTH nets each. Full factorial analysis and analysis of variance indicate that all three factors had significant influence on PTH fatigue life, but no two-factor or three-factor interactions were found. Metallurgical analysis reveals that the failure mechanism is barrel cracking near the internal pad. This mechanism has been illustrated by a finite element analysis in this study and correlated by a SEM stereoimaging analysis in the literature. The increase of electrical resistance with thermal cycles correlates well with an analytical barrel crack model. The crack length in each net at specific cycles is calculated, but fails to match predictions from a fracture mechanics model.

Cooperative application of holography and finite element modeling in determining critical vibration behavior of printed wiring boards

A PWB (printed wiring board) with dimensions of 6.5/spl times/5.5 and attached with several bolts along the perimeter as constraints was acoustically vibrated and studied using CAH (computer-aided holometry). The frequency and shape of the first (primary) and second resonant modes, revealing the full field displacements and strains of the PWBs, were recorded automatically. Zero-displacement regions were also recorded for possible locations of strategic components. Finite element (FE) modeling using eigenvalue calculations was applied to correlate with the CAH results. Adjusting the boundary conditions in the modeling effort so as to match the experimental results proved to be the tricky task in the FE analysis since the mode shapes were extremely sensitive to boundary constraints.<<ETX>>