With funding support from ONR's Navy ManTech program and in partnership with Georgia Tech Research Institute, Penn State University , Marquette University , AMCOM, Rockwell Collins, and Raytheon, the EMPF has initiated the Electronic Miniaturization for Missile Applications (EMMA) Program. The mission of this program is to introduce commercial-off-the-shelf (COTS) components into the Navy's Standard Missile guidance system.

Commercial components are getting increasingly smaller due to the miniaturization efforts of commercial component manufacturers. As a result, these components often have more functionality per given space than their predecessors. For example, the Standard Missile guidance system, which required six circuit cards, may be reduced to one circuit card using fine pitch components.

To successfully introduce fine pitch COTS components into a high reliability environment, assembly and inspection producibility issues needed to be identified. This is accomplished by building hardware with a wide range of components and documenting the manufacturing process.

The EMMA program team developed a test vehicle (Figure 1), which will use a wide range of fine pitch components, board materials and board finishes, (Table 1) to determine assembly and inspection producibility issues. Part of the team's strategy was to build the hardware while documenting the manufacturing process that would identify any assembly and inspection producibility issues. Once the hardware has been built, long term solder joint reliability will also be evaluated by performing thermal cycling and vibration environmental stress screening.

During the production run, several lessons were learned while building the hardware. These lessons occurred during the screen printing, component placement, reflow soldering, and inspection operations.

In the screen printing operation, the stencil's aperture ratio was identified as a critical parameter for good solder paste application onto the circuit card. The aperture ratio is defined as:

It is recommended that the Aperture Ratio < 1.5. Non-compliance to this guideline will cause solder paste print defects due to the poor release of the solder paste onto the circuit card. The solder paste particle size is something else to consider because solder paste particles that are too large will result in clogged stencils and print defects (Table 2).

Due to the size of the fine pitch component, tacky flux was used to coat the flip chip, as opposed to dipping the flip chip in solder flux. Additionally, screen printing solder paste for these fine pitch components would be impractical because of the small aperture size. Screen printing may cause bridging defects between the solder pads.

Attention also needs to be paid to component placement equipment capacity, with respect to the types and quantity of component feeders. The number of available feeders is based on the quantity of different component types and the types of feeders available on the component placement system. In some cases, the assembly requires two passes through the component placement process.

A lack of adequate component placement feeders could also be an issue concerning the solder paste. The time between component placement passes must be kept at a minimum to assure that the solder paste and the tacky flux do not dry out prior to the subsequent component placement passes. In this case, the EMMA team observed that water soluble solder paste was more sensitive to time than low residue/no clean solder paste during the production run.

The thermal profile is also critical to the reflow soldering process. The thermal profile should be based on a component's thermal mass. The higher a component's thermal mass, the more heat is required to bring the component to reflow soldering temperatures. With the wide range of components used in this experiment, there was a wide range of thermal masses. Therefore, a compromise had to be reached to assure that adequate heat was provided to the components with a high thermal mass, while at the same time assuring that small thermal mass components were not stressed by the reflow soldering temperatures.

Inspecting the hardware required different strategies for the various components. A microscope was used to inspect components where the solder joints were visible, such as on the quad flat packs and the small outline transistors (SOT). However, the solder joints of the ball grid arrays, chip scale packages, and flip chips were not visible because their solder joints were underneath the component and were out of view from the inspector. This required the use of an X-Ray system to assure that there weren't any solder defects, such as solder bridges and opens. Testing was performed to assure continuity within the daisy chain circuits.

As previously indicated, the EMMA test vehicles are being used to determine solder joint reliability for various fine pitch electronics packages soldered on boards with various board materials and board finishes. The test vehicles will undergo thermal cycling environmental stress testing, per the JEDEC test method JESD22-A104-A temperature cycling, using Condition B (-55oC to 125oC). Vibration environmental stress testing will also be performed using the JEDEC test method JESD22-B103A vibration variable test. Upon completion and analysis of these tests, the results will be published in the Technology Applications Guidelines (TAG) Handbook. Watch future empfasis publications announcing the release of the TAG Handbook.