Page 44 - Chip Scale Review_January February_2023-digital
P. 44
The strain relief allows some deformation to accommodate to the top of the module lid cover plate or to the stiffener ring
pulls and thermal loads, as well as to protect the tight sub- that surrounds the module. The ribbon attachment and latching
micron level alignment at the photonic die. In our case, the within the module relies on the dispense of a hard ultraviolet
assembly can rely on the self-aligning V-grooves to provide (UV) adhesive that holds the ribbon jackets in place. As the
strong anchoring points for the fibers and accommodate fiber attach solution needs to be compatible with microelectronic
bending caused by package deformations. environmental stressing and solder reflow [9,10], we will use
those load cases in the simulation. Furthermore, we use the
Methodology ribbons’ axial and lateral pulls to analyze how the package
A multi-fibers ribbon integration inside a photonic co- design around the fiber can help to relieve strain for the
package is studied here in which the strain and stress package integrity.
distribution against various fiber layout geometries within The pigtail strain relief on the lid configuration is shown in
the module are compared [1]. We will evaluate how the strain Figure 2. This architecture connects the fibers at the PIC and at a
relieving structure affects the stress with different load cases supplemental anchoring location for the fiber ribbon near the edge
(thermal and mechanical) and how the optical fiber can bend of the module. In the test module studied, the fibers are attached
within the package to reduce stress. The resulting forces at using an adhesive bridge that is in contact with both the substrate
the fiber connections to the PIC are compared to experimental and the lid, while in a second configuration studied, the anchoring
fiber pull data. The module includes an array of optical is done on a stiffener. Constraining the fibers in this configuration
communication fibers aligned with V-grooves and secured induces thermal stresses due to differences in thermal expansion
to the photonic die using an adhesive that inherently creates coefficients of the materials as they are exposed to curing, solder
a fillet with the fibers at the die’s edge. The fibers are then reflow and environmental conditions. It is, therefore, necessary to
encased within a ribbon coating (either flat or bundled) that evaluate how the fibers behave under these conditions, especially
is itself anchored inside the photonic module at a specified in terms of stress, curvature, and internal force as they are
location where bending can be induced by changing the height transferred to the PIC V-groove interface. With both architectures,
of the ribbon. A typical arrangement is shown in Figure 1. it is possible to drastically affect the fiber ribbon exit angle by
bending or machining an angle where the fibers are attached, as
shown in Figure 3. The lid cover plate or stiffener helps to limit
module warpage, and also provides flexibility to attach different
parts using fastening methods, and can even be extended to
accommodate further lengths.
Figure 1: Photo of a co-package photonic module [6] with two pigtails, where the
ribbon strain relief is created with adhesive on the package stiffener.
The advantages of using transition boots and ribbon sleeves
surrounding the fibers in the pigtail retention solution will also
be evaluated.
The stress at interfaces and bending radii control are some
of the critical aspects of the optical fiber layout inside the
package [8]. Two attachment approaches are compared and
shown in Figures 2 and 3, where the ribbon can be secured
Figure 3: Photonic device with pigtail strain relief is the attachment on the
module’s stiffener.
We propose to track the fibers’ bending and the forces
applied at the photonic die coupler for various lengths and
geometries. The initial bending will help transfer the thermal
strain from axial strain in the fibers to a bending that accepts
deformation and strains that occur with the ribbon. The
bending also allows the accommodation of the strain relief
deformation that occurs when pulling the ribbons. We did
examine the effects of the following geometric parameters: the
free fiber length, vertical offset of anchoring height, exiting
angle, and boot shape and location. Figure 4 shows how the
variable geometric parameters are defined. We will compare
the benefits of various cases and track radii of curvature of the
Figure 2: Photonic device with three pigtail ribbons exiting the module; also ribbon and the stresses at the interface.
shown is the ribbon strain relief between the lid and substrate.
42
42 Chip Scale Review January • February • 2023 [ChipScaleReview.com]