Plant, Robert J.
(2024)
Optimising laser stereolithography for constructing electronic
packages onto novel substrates.
PhD thesis, University of Nottingham.
Abstract
Additive manufacturing (AM) has matured from its initial concept as a prototyping technique to an established industrial manufacturing process for which compliance with relevant standards is required. Here, we investigate inserting components into geometries constructed directly onto non-standard substrates using stereolithography (SLA), for the purpose of electronics packaging. Silicon nitride is of particular interest as the interfacial material when packaging semiconductors and technologies such as silicon interconnect fabric. Compared to conventional encapsulation processes, SLA avoids elevated temperatures and stresses while permitting much greater flexibility to arrange components in three dimensions. This can enable an increased feature density, optimised packages for confined spaces, functional packaging to complement the operation of the device, and enables rapid production for bespoke applications.
One of the key challenges is the ability to bond the product to the substrate sufficiently to adhere to the industry standard. Additionally, interactions between the SLA process, the photopolymer and the substrate can result in distortion and compromise the ability to deliver products to the required tolerances.
To support this initiative, relevant literature has been reviewed to determine current knowledge and the gaps to be filled through further investigation. From doing so, active adhesion mechanisms were identified and methods to enhance them explored. Moreover, novel experimental processes had to be developed to produce suitable test samples. Characterising the substrate and photopolymer materials allowed potential changes in properties during the curing process to be determined, and comparison with conventional adhesion models. Furthermore, the shear stress generated from shrinkage during post build curing (PBC) has been measured to make a significant contribution to the stress at separation on untreated silicon nitride. The investigation concluded that the application of a TMSPMA monolayer to the plasma treated substrate, combined with PBC, substantially increases the strength of adhesion to an extent compliant with the industry standard and above the cohesive strength of the polymer.
In addition to adhesion, the influences of process parameters are analysed, and their potential to distort the beam and the resulting product. Distorting effects investigated include divergence, ellipticity, refraction, reflectance, over-exposure, and low intensity noise present in the beam. These have been modelled with consideration given to the influence of superposition and the machine architecture. A non-linear relationship between distorting effects and the dimensions of the build area is identified which has implications for the scalability of production. This allows the extent to which the build area can be increased, until the combined influence of these distorting effects compromises the ability to meet manufacturing tolerances, to be determined.
Further modelling supported by purpose designed experimentation, revealed the potential for significant distortions from refraction and reflectance. If sufficiently energetic, reflections from the substrate can produce spurious curing, and distort the product. Modelling the limits of exposure at which the onset of spurious curing and distortion occurs, allows an operating window to be obtained, within which the construction process can be optimised. This allows for significant savings in construction time with a reduction of 26% demonstrated.
Methods to mitigate distortion by optimising the beam’s focal point, the design of parabolic mirror profile, anti-reflective coatings, modulating the laser power, and the potential to modify the photopolymer, are reported.
Lastly, by building on prior work, a process to insert multiple large and complex geometries into the SLA build process, and to connect the installed components electrically for the construction of 3D electronic packages, is demonstrated.
It is concluded, by using the processes described, electronic packages can be constructed directly onto silicon nitride using SLA and meet the required standard of adhesion. Moreover, by applying the tools developed, the process can be optimised for scale and time, while complying with the necessary manufacturing tolerances.
| Item Type: |
Thesis (University of Nottingham only)
(PhD)
|
| Supervisors: |
Tuck, Christopher
Hague, Richard
Wildman, Ricky |
| Keywords: |
Stereolithography, Substrate, Insertion, Adhesion, Distortion |
| Subjects: |
T Technology > TS Manufactures |
| Faculties/Schools: |
UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering |
| Item ID: |
77441 |
| Depositing User: |
Plant, Robert
|
| Date Deposited: |
19 Jul 2024 09:25 |
| Last Modified: |
19 Jul 2024 09:25 |
| URI: |
https://eprints.nottingham.ac.uk/id/eprint/77441 |
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