GBIC >> MCM >> Design - Analysis

Design - Analysis
The resulting physical design of the MCM must do much more than simply fit into the customer's allocated space. The MCM has both electrical and mechanical performance requirements which must be met. Confirmation that the MCM design meets those requirements can be done through analysis and through test. This page describes the analysis options.

Mechanical Electrical

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Mechanical

The mechanical requirements placed on MCMs vary significantly - from commercial to military environments. Often, previous analysis on MCM designs or processes give enough insight to allow the MCM designer know that the MCM can satisfy customer requirements.

But because MCMs are typically custom designs, the following analyses must be usually be performed to confirm that the MCM design will meet requirements.

Thermal Analysis (temperature)
Thermal Analysis (stress)
Substrate Warp
Shock Covers device attach and wirebonds.
Vibration Covers device attach and wirebonds.
PTH/Via deformation/fatigue

Solder joint deformation/fatique Is often considered the responsibility of the next level of assembly
Lead Strength

Die Shear Strength

Lid Deflection

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Electrical

Because of the relatively high cost of MCMs, customer typically attempt to complete their products using discretely package devices. In support of that, they typically create a breadboard of their design to confirm electrical operation of the schematic. Analysis performed by MCM engineers typically begins with the assumption that the basic design works and is targeted on determining how the packaging approach affects the design performance.

Timing
Timing analysis is geared towards determining the delays experienced by signals between two points. Signals move along traces at predictable speeds, primarily as a function of the dielectric constant. In practical terms, this means that a timing analysis equates to calculating the length of all traces. This information is translated to time delay and compared to the requirements as provided by the customer.

A point to remember is that the shortest possible trace (no delay) is not always required. Since correct signal timing involves both rise times as well as hold times, designs may operate under both minimum and maximum signal delay times - and which may be different for each trace.

In special cases, the timing requirements may involve matching time delays between signals or within groups of signals. This correlates to specifying line lengths between specific signals - often to within less than 100 mils. This can place significant demands on a router, resulting in significant increases in layer count.

Signal Integrity
A general goal in signal transmission is that the signal at the receiving end replicate the signal that was transmitted - signal integrity. In most designs, there is always some distortion, arising from several sources.

Energy loss
Reflections
Crosstalk
Power supply noise

EMI
Almost any metallic surface acts as both a receiver and transmitter of electromagnetic radiation. The goal in MCM design is to minimize those effects. Fortunately, in most designs the effects are small and special design techniques are not required. However, as frequencies move above 100MHz the effects become more pronounced. At 500MHz and higher, it is almost always necessary introduce design controls to manage the EMI effects.

A special case under EMI is the effect that floating metal planes (such as the MCM lid) can have. Such planes can act as excellent antennas and specific actions are needed to isolate the received signals from the MCM.

Resistor Design
In many cases, the use of integrated (or embedded) resistors is used as part of an MCM design. This approach minimizes the size taken up by the devices and allows customization of the design values. Designing the physical dimensions of the resistors, in order to achieve the correct resitance value, is one of the analyses performed by the MCM engineer.

Design-for-Test
At the time that MCMs are manufactured, they typical go through an electrical test to confirm their operation. Additional tests may be needed to troubleshoot failed MCMs in order to determine what repair actions are needed.

Such tests require not only access to key I/O within the MCM design, but may also require the introduction of additional circuitry to interface with the test equipment and test software. It is also typical that some test points (for troubleshooting purposes) may be brought to the surface of the MCM interconnect, but not brought out to the MCM I/O (leads or BGA pads). Introduction of these test points can have negative effects on the MCM performance, so their impacts must be evaluated carefully.