By Lee R. Levine, Contributing Editor
Companies experienced in packaging conventional ICs will find they have entered a different arena when they enter the world of optoelectronics assembly and packaging. Photonic Devices Photonic transmitters and receivers are often produced as "butterfly" packages. Figure 1 is a sketch of a typical package containing multiple bonding surfaces with different metallizations and a large height difference between bonding surfaces. These features increase package complexity and challenge wire bonding. Other packages, such as amplifiers, may be long and narrow. In general, however, they are gold plated rectangular metal cans with cavities. The cavities contain the component circuitry for their optoelectronic function. Bonding is within the cavity to the components, and to the cantilever leads projecting into the cavity from the cavity walls.
Vibration Cantilever leads (leads protruding through the package wall like diving boards), can vibrate and attenuate ultrasonic energy. In fact, research at Delco Delphi has shown that if the natural vibration frequency of the lead is greater than half the ultrasonic bonding frequency, the lead will resonate, and bonding will not occur.* Shorter cantilevers and stiffer beam cross-sections can improve the bonding process. However, bonding very close to the package walls is required, and normal capillary designs are unable to bond this close without mechanical interference. Bottle-necked capillaries and other specially ground capillary designs are available for deep access bonding near the package walls, but are more fragile than standard designs. Opto Components Many opto packages contain some common components. For example, lasers and PIN diodes, which are normally mounted on a silicon platform with the ribbon bond making the connection to the silicon, are present in most receivers, transmitters and transponders. (Figure 1). LTCCs The silicon platform is, in turn, mounted to an LTCC (low-temperature, co-fired ceramic) circuit board. Depending on design requirements, there may be multiple LTCCs at different heights or locations within the package. The LTCC is a densely integrated, multi-layered circuit board that provides the mechanical stability required for good, long-term optical performance. Other common optoelectronic package components may include TEC (thermo-electric coolers), super- semi optical isolators (SSIs) and ICs. Solder Hierarchies Optoelectronic packages are often assembled with a hierarchy of solder used for die and substrate attachment. Solder hierarchies are a series of solder alloys that melt at progressively lower temperatures. As different components within the package are assembled, each new solder reflow cycle must melt and reflow to attach new components without reflowing those components already assembled. Optical designs require very precise placement and control of the light path within the package to achieve a high efficiency optical response. If assembled optical components reflow a second time they will lose their placement precision and lower package reliability. The wire bonding process temperature must not exceed the lowest reflow temperature (often as low as 130°C), because wire bonding occurs at the end of the assembly process after the lowest melting temperature solders have already been reflowed. Individual components within the package (upper level LTCC substrates) can often be wire bonded at 100°C. At this low temperature, wire bonding requires high frequency ultrasonic generators, and the devices must be plasma-cleaned prior to wire bonding. The cleaning will increase the wire bond pull strength significantly and often allows high yield manufacturing, where otherwise unacceptable process yields would be experienced. Improving Yield and Reliability Bonding the ball on the LTCC surface can often improve yield and reliability. Since the ball is soft, newly solidified and clean, it bonds to the gold thick film surface of the LTCC readily. Second bonds on the soft LTCC surface often possess low yield. The soft thick film of the LTCC does not provide enough mechanical resistance for the wire to deform and bond during second bond. Instead, the wire is pushed undeformed into the LTCC thick film, forming a low-strength, poorly welded connection. The LTCC surface area is large and allows bonding large diameter ball bonds. These larger ball bonds provide benefits by offering a larger weld cross-section with higher strength. Process optimization poses additional difficulties when components are as valuable as some optoelectronic devices. Small lot sizes, short runs and lack of available components all make optimization, with statistically meaningful sample sizes, a challenge. DOEs are necessary to understand the complex bonding conditions. Multi-variate response surface experiments, with each metallization type grouped to find a separate optimum, are required to optimize the process.
Loop Shape The extreme range of height differential (as much as 4-5mm) within an optoelectronic package requires new equipment and tooling features (deep access). Programmable focal height for the vision (pattern recognition system) is mandatory to accurately and repeatably locate and bond components whose surfaces are at different heights. Even with this capability, PRS can sometimes be a challenge. Automatic wire bonders with high speed PRS need unique vertical and horizontal edges with good contrast. Optoelectronic devices often contain multiple levels of gold-coated components placed close together. Finding a specific cubic gold-coated capacitor against the gold background on a LTCC is a challenge when there are several in the same field of view (alias images). In IC ball bonding, the wire normally descends from the ball bond to the second bond on a lower elevation surface. Optoelectronic packages often possess wires that ascend, sometimes as much as 3mm, from the ball bond to the second bond. Figure 1 shows a sketch of a wire bond from an LTCC on the package floor to an LTCC mounted on the package ledges. The ascendant shape provides higher yield and reliability than the descendant shape. However, it is a difficult shape to achieve. Maintaining loop shape and control is difficult because bonding equipment manufacturers have not yet developed loop trajectories optimized for this application.
Ribbon Bonding Only high frequency components are interconnected with ribbon bonds (thermosonic wedge-wedge bonding using flattened gold wire with a rectangular cross section). High frequency electrical signals are conducted through skin effect. At 10GHz, the penetration depth of an electrical signal (skin thickness), is less than 1µm. A good first order approximation for equivalent conductivity between round and ribbon wire at high frequencies is that the circumference of a round wire equals the perimeter of the ribbon. Figure 2 shows the relationship between equivalent diameter round wire and ribbon. Conductivity is only one of the reasons ribbon wire is used for bonding high-frequency components. Mutual inductance and cross talk between adjacent ribbons is lower than for equivalent round wires.
In addition, the bonding parameters (ultrasonic power and bond force) required to bond a thin ribbon are significantly lower than for the equivalent diameter round wire. Stiffness of the thin ribbon cross section is also significantly lower than for its round counterpart, allowing ribbon to bend and form a loop with less force than equivalent round wire. Lower ultrasonic power and bond force combined with easier bending and loop formation are better for bonding fragile high-frequency die that are often made from brittle materials (GaAs, LiNb3). Additionally, ribbon bonding can be performed on high-speed wire bonding equipment, rather than slower heavy wire bonding equipment. Conclusion & Summary Unlike traditional semiconductors, optoelectronic device packages are usually assembled with a "hierarchy" of solder used for die and substrate attachment. Moreover, process optimization involves additional challenges over IC packaging, when components are as valuable as some optoelectronic devices. Small lot sizes, short runs and lack of available components all make optimization, with statistically meaningful sample sizes, a challenge. * R. Bibby, J. Hearn and M. Webster, "Developing Interconnect and Connector Technologies for a Hybrid Engine Control Module," Proc. ISHM 1994, p. 462.
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