Thermal Deformations Observed in Leadless Ceramic Chip Carriers Surface Mounted to Printed Wiring Boards

Abstract

Leadless ceramic chip carriers and other surface mountable components can be soldered to printed wiring boards. It is wellknown, however, that temperature changes and temperature gradients can cause large strains in the solder joints. What is not widely appreciated is that the joints are not in simple shear even on a macro scale, much less on a micro scale. We consider three modes of joint deformation, characterized by the disp!acement of the pads (or lands) to which the solder is attached. Mode A, which is "shearlike," is characterized by a difference in the in-plane displacements of the two pads. Mode B, associated with bending, is characterized by a change in the angle between the planes of the two pads. Mode C, which is "tensionlike," is characterized by a difference in out-of-plane displacement of the two Pads. Any given thermal change Will produce a superposition of these three modes of deformation. These deformations were measured using strain gauges and holographic interferometry, two techniques which complement each other in several ways. A strain guage measures in-plane displacements, giving modes A and B; it measures only one part of the sample; and it is suitable for thermal chamber or power cycling. Holographic interferometry measures out-of-plane displacements, giving modes B and C; it measures every part of the sample; and it is most suitable for power cycling studies. In thermal chamber cycling, we find that above 50°C the chip carrier and the board expand as if they were independent. At lower temperatures, the considerable tractions that develop cause board bending (mode B), as well as nonlinearity in the temperature dependence of the mode A deformation. The board tends toward the chip carrier for decreasing temperature. The bending causes any given row of solder joints to have a distribution of mode C deformations, some tensile and some compressive (corner joints are in tension). When power is applied to a chip carrier mounted on a printed wiring board, the average temperature rises, causing mode A deformation similar to that in a thermal chamber. The temperature gradient through the thickness Of the board, however, causes the board to bend toward the chip carrier, overcoming the effect of the traction on the joints, which is in the opposite direction. All three modes of deformation are proportional to the power. Mounting of the printed wiring board has a strong influence on modes B and C. Rigid clamping of the board decreased modes B and C deformation by a factor of two compared to that of a board with free edges.