Introduction
Outsourced Semiconductor Assembly and Test (OSAT) companies provide third-party IC packaging and testing services, optimizing costs, yields, and cycle times by leveraging economies of scale through high volume production. Traditionally, OSATs handled IC packaging through processes like wire bonding and flip chip assembly, as well as IC testing like optical power measurement and high magnification inspection.
However, with the growing silicon photonics market, large OSATs are now integrating advanced optical assembly capabilities alongside traditional IC packaging within the same facilities. This allows OSATs to offer streamlined, turnkey photonics assembly services - but also introduces some unique challenges that must be addressed.
The Photonics Assembly Challenge
Compared to standard IC assembly, integrated photonics packaging involves many more variables and degrees of freedom. Just a few examples:
Deciding on a laser source (O, C or L band) based on required intensity and polarization
Selecting coupler type, structure, and pitch
Specifying wavelength, polarization, optical intensity, and index of refraction
Choosing fiber type and connector format
The increased complexity means photonics assembly requires sub-micron placement accuracy on 3-6 critical axes. Automating this process is difficult - current placement machines are mostly custom solutions, while reconfigurable platforms tend to be lab-grade and unsuitable for high volume manufacturing.
Furthermore, the multi-disciplinary nature of photonics packaging means process development and optimization can take 1-20 weeks for each new design of experiment (DOE). Expertise is needed in optics, electronics, materials science, and algorithmics to design functional active alignment routines.
Material Challenges
Standard IC assembly uses mature, well-defined materials like wire bonds and solder balls. But for photonics, there are far more BOM variations and compatibility constraints:
Different PIC foundries and coupler types require different epoxy adhesives with specific refractive indexes
Multiple (2+) epoxies are often used in the optical path to reduce light loss
Fiber array variables like pitch, type, number of fibers, polarization, thickness, polishing angle, connector, and protrusion must be carefully matched
These frequent BOM changes make it harder for OSATs to stockpile materials and benefit from economies of scale purchasing.
Tooling and Process Challenges
Active alignment systems require specialized, reconfigurable tooling:
Fiber array pickup tools
PIC holders
Monitor photodiode probers
Sources and photometers for different wavelengths
Adjustable cameras, UV lamps, and voltage probes
The alignment process itself must also be reoptimized for each new product:
Fiducial marking and alignment algorithms
Step sequences and tolerancing
UV and thermal curing profiles
These non-recurring engineering (NRE) costs for tooling and process development are a significant handicap for OSATs trying to ramp photonic production.
OSAT Photonic Packaging Flow
To illustrate the scope, here's an example photonic packaging flow that an OSAT like Silitronics might execute:
This package contains:
60x60mm built-up substrate
25x25mm PIC with 15,000 bumps
2 fiber array units (FAUs) with 12 fibers each
2 ICs with 20,000 copper pillars each
The entire NRE and initial DOE are done in-house at the OSAT's facility (which provides IP protection, automated capabilities, and ISO certification). Only then is the product transferred to higher volume manufacturing within the OSAT.
The overall process flow is:
Design and signal integrity work (4 weeks)
Substrate fabrication (10-14 weeks)
Die preparation like dicing (4 hours)
Flip chip assembly (2 days)
FAU fabrication (1 week)
PCBA (1 day)
Active fiber alignment (1-5 weeks)
So for a complex photonic product, the cycle time from design to initial manufacturing can be 3-5 months at the OSAT.
The OSAT Solution
To summarize, some key challenges for OSATs providing photonic assembly services include:
Large number of variables and critical axes for alignment
Frequent lack of standardization and material/process reuse
Multi-disciplinary engineering expertise required
Substantial NRE costs for tooling and process development
To overcome these challenges and offer robust photonic packaging capabilities, OSATs like Silitronics are:
Investing early in advanced equipment for wafer-level testing, active alignment, and final optical measurement. This preparation happens before high volume demands materialize.
Establishing close partnerships with customers and leading technical collaboration from day 1 of the design cycle. Co-developing solutions in-house accelerates understanding and streamlines future programs.
Committing substantial engineering resources to developing alignment technology, processes, and computer vision/algorithm expertise specific to integrated photonics.
While difficult, mastering these disciplines is critical for OSATs to provide truly turnkey silicon photonic packaging services as the market continues growing rapidly.
By proactively addressing the unique photonic assembly challenges through strategic investments and deep technical collaboration, OSAT leaders like Silitronics are positioning themselves as true one-stop partners for the next wave of integrated photonic products.
As Dr. Dhiraj Bora, CEO of Silitronics, stated: "We've made several investments to establish OSAT leadership for photonic packaging in the United States. These have already resulted in wins with hyperscalers, Tier 1 OEMs, and leading AI/ML/cloud technology startups. OSATs must invest in engineering and technical capabilities before high volume demands materialize. With our advanced capabilities like automated PIC testing, active alignment, and final optical measurement, Silitronics is your true one-stop OSAT partner for integrated photonic and IC assembly."
Reference
[2] D. Bora and C. Hibbs, "Integrated Photonics Assembly Challenges for High Volume," presented at the Silitronics APC Webinar, Jan. 10, 2024.
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