Quality and Reliability

As was discussed in the first section and shown in Table 12.1, the flat world view of integrated circuits and microcircuits does not fully address the MEMS packaging schemes. However, failure mechanisms seen in the microcircuit world may be evi­dent in the MEMS realm. Common failure modes seen in microcircuit packaging are:

Die and passivation cracking

Delamination between the die, die attach, die pad, and plastic passivation

Fatigue failure of interconnects

Fatigue fracture of solder joints

Warping of printed circuit board

Most of the failures as described above are due to the following sources:

Mismatch of coefficients of thermal expansion between the attached materials.

Fatigue fracture of materials due to thermal cycling and mechanical vibration.

Deterioration of material strength due to environmental effects such as mois­ture.

Intrinsic stresses and strains from microfabrication processes such as thermal oxidation, diffusion, and depositions [38].

Suffice it to say that the MEMS packaging engineer must consider all of the above and then more.

12.10.1 MEMS Packaging Reliability Concerns

As discussed in earlier chapters, virtually all MEMS devices require micro-fabrication techniques such as surface micromachining. All future processing steps including bonding depend on proper surface treatment prior to actual bonding. Any improper treatment in surface conditioning would leave voids at the bonded inter­faces. These imperfections can act as crack initiators or stress concentrators for subsequent delamination of the interfaces [38]. Any improper selection of materials will be apparent in the temperature dependent mismatch failures. Thus, precision control of all processes e.g. temperature, contact pressure and applied voltages in anodic bonding process is fundamental to successful packaging scenarios. Once packaged, the device sees reliability and quality assurance screens and applica­tion stresses. Any imperfections at the interfaces and delamination of the bonding surfaces will be exacerbated and may lead to failure.

At all fabrication steps, the concerns of material compatibilities need to be addressed. During packaging of MEMS, stresses will be distributed within the die attachment, die and substrate the reliability of the packaging structure. Numerous studies in the literature discuss stress during processing steps [39].

Outgassing may cause environmental contamination that leads to clogging or material build up. The device may then become inoperable depending on its function. Contamination binding and build up have been found to cause device failures in strategic active areas [40].

The inner surfaces of micro-conduits used for electro-osmotic and electrophore-sis pumping require coats of special polymers, allowing for the release of ions under the influence of electrical fields. These ions interact with those released from the contacting fluids via electromigration and thus results in fluid capillary flow. These delicate thin film coatings, often a few nanometers thick, can be attacked by free ions, reducing their effectiveness over long electric field exposure times. This reliability issue is difficult to resolve [38].

Multiple stresses may be more detrimental to reliability than the effects of a single factor. In the design process, both design factors and test criteria must con­sider both individual and/or combined life cycle stresses to produce the robustness needed to withstand the hazards identified in the system profile. The synergistic effects of typical combined environments can be illustrated in a matrix relationship, which shows combinations where the total effect is more damaging than the cumu­lative effect of each environment acting independently. For instance, an item may be exposed to a multitude of environmental factors such as temperature, humidity, alti­tude, shock, and vibration while it is being transported. Demonstrating adequate end of life service must include combined effects. For example, many delicate MEMS device components, such as the thin silicon diaphragms used in micro pressure sen­sors, are in contact with corrosive or reactive gases whose is to be sensed. Many of these gases, such as the exhaust gases from internal combustion engines, are hot and contain corrosive chemical compounds. Extended exposure to these hot media may cause serious damage to delicate components [38].

Date: 2015-02-28; view: 1041