Precision molded data link package earns parts, assembly and test savings
An integrated lens/receptacle eliminates the need for a header assembly by integrating all electronics and active opto-electronic devices on one circuit board
A.J. Heiney, C.L. Jiang and W.H. Reysen
Data link packaging efforts to date, including low-cost packaging, have primarily concentrated on integrated circuit packaging improvements and consolidation. Industry efforts have consistently relied on the proven transistor-outline header to house active devices and to mate with such optical coupling elements as gradient-index lenses and split sleeves. The header may also house preamplifier circuits. Although this approach is technically sound, it shifts the cost from the drive/receive electronics to the header-based active device mount.
The packaging approach for a new molded lens/receptacle reduces data link manufacturing costs through optical solutions to the active device mount problem. An integrally molded lens/ receptacle element receives the fiber, and couples and bends the light to the board. It eliminates the need for headers by allowing tight integration of all electronics and opto-electronic devices on a pretested board assembly.
Precision polymer molding of the lens/receptacle decreases parts count and assembly labor, streamlines testing and allows use of modular connector assemblies. This packaging approach can accommodate data rates in excess of 600 megabits per second.
In the conventional 16-pin data link, the electronic assembly and an optical subassembly are separately built and mated. The electronic assembly consists of integrated circuits and passive components mounted on a co-fired ceramic substrate. A Kovar subframe is brazed to the bottom of the ceramic for attaching the case frame.
This conventional optical subassembly generally has a stainless-steel fiber receptacle that accepts a TO-lensed assembly. The opto-electronic device--light-emitting diode or p-type-intrinsic-n-type detector--is die-bonded to a ceramic submount that is attached to, but electrically isolated from, the header. The opto-electronic device and the top of the submount are then wire-bonded to the header leads. A sapphire lens cap, welded to the header, seals the opto-electronic device and provides coupling.
The header assembly must then be actively aligned into the receptacle and fixed with epoxy. Next, the manufacturer uses a clip to mate the tested optical subassembly to the case frame and then solders the leads to the board. A lid provides bubble-tight sealing. This package provides a rigid and stable environment for the data link.
In the construction of the new data link design, a molded lens/receptacle replaces the header, providing the seat for the fiber and the board. It conducts light to and from the opto-electronic device on the board, bending the light at a 90-degree angle. The six-layer printed circuit board is assembled with the same circuit and components as the conventional design, using chip-on-board assembly. The PIN or LED opto-electronic device is then die- and wire-bonded onto special board traces, which provides low capacitance and high thermal conductivity.
In this configuration, the opto-electronic device and the populated board simultaneously undergo burn-in. The lens/receptacle is then attached to a case, and the board is actively aligned within the case.
The new packaging approach cut in half both the number of parts and process steps. This production savings also resulted in several production advantages:
Testing carried out earlier in the integration path.
Reduced parasitic capacitance leading to higher gain and ultimately higher yields.
Decreased electromagnetic interference relative to the long exposed leads of a TO header.
Easier gripping for active alignment compared with headers.
The key to cost reduction in the new package is the integration of functions within the molded lens/receptacle. The bore accepts standard 2.5-millimeter diameter ferrules, with a total tolerance on the bore dimension of 0.007 mm. This tight range provides adequate resistance to fiber movement within the bore so optical coupling is not compromised.
The outer diameter of the bore nose conforms to the Japan Industrial Standard and Low-Cost Fiber Distributed Data Interface specifications for the protrusion between the ears of the SC-style connector. With this conformance, the lens/receptacle can be used for several connector types. For example, the lens/receptacle is currently used for ST and SC duplex and FDDI media interface connector interfaces.
A fiber stop is inserted into the bore to provide repeatable optical spacing of the fiber to the first lens surface. This step is performed before the lens is mated to the case. A shelf that runs the width of the board is integrated into the lens/receptacle to provide repeatable optical spacing of the board (and, thus, the device) to the second lens surface. During active alignment, the board rests on a shelf in the rear of the case and on the lens in the front of the case.
Molding of the bore and first lens surface is accomplished by using the same precision core pin. This process ensures the optical axis of the lens surface is highly concentric with the bore, and eliminates concerns with the off-axis performance of aspheric surfaces.
The second lens surface is smaller in diameter than the first because the beam expands inside the lens while traveling from the second to the first surface. The 45-degree prism that folds the light path is a total internal reflection surface. Reflection of a non-collimated beam is ensured by selecting a material with a relatively high index of refraction.
Other criteria for the lens material selection includes resistance to high temperatures, chemicals, galling and wear, along with flame retardance and low absorption and scattering around 1.3 microns. The precision of the lens/receptacle case mating surfaces enables assembly repeatability.
A custom ray-tracing simulation program is used to prove the design of the lens/receptacle. This tool permits optimization of transmitter performance to a point beyond the sapphire sphere lens, despite the design goal of eliminating anti-reflection coatings.
To accomplish this ray-tracing work, simulations for a 45-degree prism and aspheric surfaces are added to an earlier package model. The new model generates 160,000 rays emanating from the LED active area with a Lambertian distribution. The ray-tracing program then progressively calculates the refracted or reflected angle and Fresnel reflection losses for each surface.
The program also simulates the gradient index multimode fiber. Coupled power is calculated by taking the ratio of the guided rays to the starting number of rays and considering the surface reflection losses. The steps are repeated for each off-axis position of the LED to obtain the lateral coupling curves. The optical ray-tracing model shows good agreement with experimental results.
Comparing the performance differences between the standard and new data links reveals that preamplifier input capacitance is reduced; this capacitance reduction permits increased gain or, if necessary, larger PIN detector active area for increased optical coupling tolerance. The PIN coupling efficiency is experimentally verified to be approximately 80%, which is 10% less than the conventional design because of the lack of an anti-reflection coating.
However, the improvement in gain more than compensates for the reduced coupling efficiency. Losses from aberrations were verified to be low by comparing coupling using a PIN with a larger active area.
Reduced parasitics in the connection of the LED to the drive circuit result in faster edge speeds and wider bandwidth, yet the overshoot remains within the same guidelines as the standard product. Spectral width at room temperature is narrower, which increases center wavelength tolerance and improves yields for compliance with FDDI standards. More important, because the electrical design is temperature compensated (drive current increases with temperature), the spectral width reduction from the conventional data link at 70C is approximately 25 nanometers.
Future enhancements are expected to improve placement accuracy of the opto-electronic device and to widen the lateral width of the optical coupling. At some point, these improvements will eliminate the need for active alignment. The board and lens could then be snapped into place in the case, further reducing cost and processing time. u
A.J. Heiney is a design engineer, C.L. Jiang is a member of the technical staff and W.H. Reysen is manager of development engineering at AMP Inc. in Harrisburg, PA.