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E-band cost, reliability concerns in MMIC packaging

Posted: 15 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:RF systems? semiconductor packaging? E band? millimeter-wave? microstrip?

Traces and other features are formed in conventional PCB fabrication by coating the copper with a photoactive etch resist, selectively exposing the resist to light where the copper should be retained, then etching away (subtracting) in an acid bath the unexposed resist and the copper underneath. The problem is that the bath not only dissolves the copper in a vertical direction but also eats away some horizontally from the trace walls, especially at the top, resulting in traces that are trapezoidal in section. As a result, the smallest tolerance for trace width that can be reliably achieved in subtractive PCB fabrication is within 0.5 mil of design value. When you are dealing with millimeter-wave systems and fine traces that run for anything beyond a very short distance, a deviation of plus or minus 0.5 mil results in impedance variations that are certain to cause signal degradation.

We at Rosenberger North America learned, however, about an alternative, semi-additive PCB fabrication process that produces square-edge traces as fine as 1.25 mils wide over long distance with virtually no deviation from as-designed dimensions. The process is in place at an established Silicon Valley PCB manufacturer, Sierra Circuits.

System test results
To successfully implement microwave and millimeter-wave products, all elements within the system must ensure precise impedance control. The impedance of microwave connectors, CPW traceslocally within the circuit itself, and globally across a PCB panel C and connections within the IC package must all be controlled tightly. Rosenberger has been testing the technologies described here, as well as the related microwave connectors, using prototype boards running at frequencies up to 80GHz to see how well the additive PCB process would complement the millimeter-wave die packaging.

The evaluation boards incorporated Rosenberger solderless, microwave PCB-mount connectors, typically used in such ultra-high-frequency, test-and-measurement applications as package or module characterisation for microwave semiconductors and other product development environments. Those coaxial cable connectors are available in several versions, namely 2.92-mm (up to 40GHz), 1.85-mm (up to 70GHz), and 1.00-mm (up to 110GHz), commonly referred to as K, V and W connectors, respectively. The 1.00-mm version was used in this investigation. All the connectors were surface mounted to the PCB without solder and were secured by dowel pins. They have a clamping mechanism that accommodates a wide range of board thicknesses and provides continuous ground connection between contact pin and PCB.

Figure 2: Reflection loss over frequency for each of the three Rosenberger connectors is shown.

The results of the testing have been impressive. The semi-additive Sierra Circuits process achieves a manifold improvement in trace-width tolerance and placement, compared with subtractive fabrication. The absolute target value and the maximum impedance variation across an entire panel of boards were found to be 50.6 0.7?.

Figure 3: Impedance variation across an entire panel of test boards was found to be within 1.4?.

Tests on other panels obtained similar results. This bodes well for high yield rates for boards in volume production. Semi-additive fabrication involves fewer steps than the subtractive process, has faster throughput, and is compatible with any laminate.

End to end performance was evaluated using a time-domain reflectometer (TDR) to measure the impedance along the entire path. TDR test results across all elements of the test boards are shown in figure 4.

Figure 4: TDR test results demonstrate how well all elements, from the die to the PCB connector at the edge of a board, work in concert to maintain signal integrity..

It appears, then, that key issues affecting E-band systems for automotive applications have been resolved. With low-cost IC packaging and PCB fabrication now capable of providing controlled impedance at these frequencies, developers now have the resources they need to make millimeter-wave systems more practical.

About the author
Eric Sanjuan is Director of Coaxial Interconnects for Rosenberger of North America.

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