Optoelectronics development: with the surge of optoelectronics packaging, component alignment issues need to be resolved.

Problem

One of the many concerns unique to optoelectronics applications is the effect of adhesive deposition control and materials properties on optical alignment. One customer recently reported an unexpectedly rapid loss of alignment with a component on an optical bench, in a specific commercial product, during thermal excursions. Part of this problem varied with time and/or history; and variations in degree of severity, between parts in the same lot, suggested process control problems.

Problem Solved

FEM Isn't Always Perfect

Supposedly, the product had been designed taking effects of the expansion and compliance of adhesive layers predicted by finite element modeling (FEM) into account. However, experiments showed constrained epoxy layers becoming increasingly anisotropic in gaps below 100 mm where chains started orienting themselves preferentially in-plane. Reducing the layer thickness, from 4 mm to 1 mm, led to increases of a Factor of 6 in the in-plane modulus and half an order of magnitude in the out-of-plane thermal expansion coefficient. The latter means that the thin layer would expand at least as much, while the former would enhance thermal mismatch-induced warpage of the optical bench beyond that predicted by FEM.

Materials Are the Key

While such effects would account for differences between predicted and observed behavior it was, however, shown that some of the problems with the above product were related to irrecoverable (plastic) relaxation effects. Not surprisingly, the plastic flow properties were also found to depend strongly on adhesive thickness. In general, the sensitivity of all relevant properties to constraint is strongly materials dependent. These factors could be accounted for by basing product design and materials selection on properties measured for realistic configurations--as opposed to relying on supplier data sheets.

Even more importantly, the exercise also revealed the sensitivity of the optical alignment to adhesive location and volume. By pressing one of the components into an adhesive, during active alignment, the adhesive would wet up the sides, forming fillets that varied with adhesive volume and location. Modeling showed asymmetric fillets, on opposite sides, tilting the optical axis of the component by up to a degree for a couple of degrees centigrade of temperature change. This tilt explains a clear variability of the final products and demonstrates the need for improved control of the deposition of the nano-liter adhesive volumes involved.

Conclusion

Luckily, an ongoing investigation of the fundamentals--of one of the few approaches relevant--for automated production in this regime, pin transfer, has already allowed considerable optimization in terms of the combination of materials and process parameters. Expectations are that sufficient control, to ensure high yields, for the present product will soon be achievable.

The Universal Surface Mount Technology (SMT) Laboratory was founded in 1987 with the basic goal of providing competent technical support and process development for Universal Instruments customers' electronic manufacturing needs. DEK Printing Machines (Flemington, NJ) and Vitronics Soltec (Stratham, NH), our sister companies, also maintain surface-mount process engineers in the laboratory to provide support for their customers. The laboratory contains two complete automated production lines with all associated production related equipment. The laboratory provides failure analysis capabilities with an array of non-destructive and destructive analytical techniques.

In addition to its role as a customer support facility, tire SMT Laboratory serves as a research entity funded largely by the Area Aa-ray Consortium. The Consortium was first founded by Universal's SMT Laboratory in 1991 as the Ultra-Fine Pitch Consortium and has supported the development of tape-automated bonding (TAB), ultra fine pitch (UFP) quad flat packs (QFPs), ball grid arrays (BGAs), chip .scale packages (CSPs), wafer scape package (WSP) and flip chip attach technologies. The laboratory has also conducted development efforts in areas related to pin-in-paste and 0201 components.

The aim of the laboratory's optoelectronics packaging focus is to help bring this technology to the level of high-volume, automated manufacturing common in microelectronics packaging today. Aside from efforts on optical fiber handling, gold/tin soldering, optical coupling and alignment, as well as some product specific developments, a major emphasis of the program is the use of adhesives in optoelectronics packaging.

For more information on the laboratory, visit www.uic.com.

George Westby is the director of the Advanced Process /Surface Mount Technology Laboratory at Universal Instruments, and is the director of the Area Array Consortium; Binghamton, NY; e-mail: west [e-mail].