By Richard Dec, Merrimac Industries
EE Times
(10/09/00, 12:18 p.m. EST)
The Multi-Mix process begins with commercially available polytetrafluoroethylene (PTFE) composite, copper-clad laminate material having inherently low dielectric loss and stable microwave properties. A low Z-axis coefficient of thermal expansion (CTE), close to that of copper and aluminum, ensures excellent reliability of plated-through holes. Controlled XY-plane thermal expansion, together with a low modulus, affords excellent reliability of surface-mounted devices in the most severe thermal cycling and thermal shock environments. Further, a low and uniform thermal coefficient of dielectric constant, coupled with the low CTE, result in consistent electrical performance over a wide operating temperature range.
Complex microwave circuit patterns and transmission-line geometries may be chemically photoetched on the copper, maintaining dimensional tolerances of 0.0025 plus/minus 0.0005 inch. The incorporation of thin metal-film etched resistors can be routinely accomplished with accuracies of 5 percent. Indexing holes for layer-to-layer alignment in assembly are precision-machined or drilled into each board. Layer-to-layer plated-through holes may be realized with a minimum diameter of 0.005 inch and typical aspect ratios of 20:1. Active device attachments are embedded in cover-layer cavities to provide environmental protection and allow precap inspection and test. The pick and place of discrete components can be demonstrated on array panels up to 18 x 24 inches, with ball grid arrays.
In fabricating a bonded multilayer assembly, the stacked layers are placed in a fixture to which carefully controlled, uniform pressure and temperature is applied to meet the substrate fusion-bonding requirements. After cooling and removal from the fixture, edges are plated for EMI shielding and ground-plane integrity. Finish platings for environmental protection include immersion tin, fused tin-lead and nickel-gold.
Whether for military or commercial applications, improved EDA modeling and simulation tools have been developed for microwave and millimeter-wave circuits and packages. Integrated mathematical, electromagnetic, thermal and mechanical modeling capabilities on a modern PC-type platform allow total package analysis before actual hardware fabrication. In addition, the Multi-Mix module architecture uses a structural template that overlays outlines and makes common interconnection paths.