Relentless demand for more functional capability and less weight and volume has led to the rapidly escalating use of leadless electronic components on ceramic circuit boards in defense systems acquisition. While possibly not the only approach in electronic packaging and interconnection technology, this has proven to be the approach of choice. Electronics manufacturers are taking steps to acquire and to implement it in their designs, and in the next few years, most new military electronics will incorporate this technology.
Problems in system design and manufacturing have raised serious concerns about the inherent reliability and producibility of leadless components on ceramic circuit boards. A number of defense systems acquisition programs have required redirection following their inability to meet schedule, cost or functional objectives. However, solutions have been found for the major problems encountered, and this document summarizes the industry's current best practices. While the technology is not without risk, low risk can be assured by understanding and adhering closely to these guidelines.
The key element in the technology is the leadless chip carrier (LCC) which has terminals on all four sides but lacks leads extending from the package. While this permits far more terminals than other integrated circuit packages, attachment to circuit boards lacks the flexibility which leads would provide. Prevention of solder joint cracking under extremes of temperature requires closely matching the thermal coefficients of expansion of the ceramic-based LCCs and their circuit boards. For this reason, ceramic circuit boards of alumina or beryllia are used.
A ceramic circuit board, more precisely named a
multilayer interconnect board (MIB), starts out as a ceramic blank in the
range of 20 to 60 mils thick. Either noble metal (gold or gold alloys) or
copper is used to provide circuit interconnections. Conductive and dielectric
materials in paste form are printed and then fired on the ceramic blank in
layers to form the MIB. Layers are interconnected by "vias" of conductive
material placed in holes through the dielectric layers. To attach LCCs to the
board, solder in paste form is applied, components are placed, and joining is
accomplished by heating (vapor phase reflow) which melts and solidifies the
solder. The organization of this report is shown in Figure 2-1 and the chapters are summarized in the
following sections.
