EIPC Summer Conference 2023: Day 2 Review

Estimated reading time: 13 minutes

Update on Metrology
Uwe Altmann from KLA Corporation gave an update on metrology for advanced PCB and IC substrate manufacturing. He remarked that metrology is becoming increasingly important as PCB producers move toward more advanced manufacturing technologies like mSAP to meet growing bandwidth and speed demands. These advanced processes pose yield challenges and increase the need for metrology on fine copper lines, surface roughness, and dielectric thickness. He discussed different types of sensors that can be utilised for measurement, and described a proprietary optical profiler for PCB and semiconductor packaging on panels and substrates. 

This panel metrology system features multi-mode optics that enable simultaneous measurement of step height, film thickness, and topography by combining the capability of several tools into one compact platform, capable of measuring a variety of applications such as via and trace dimensions, bump dimensions, overlay, film thickness, roughness, and panel warpage. It could be used for direct thickness measurement of Ajinomoto build-up film. The profiler could measure both test and inline product panels, providing the data required to enable process feedback during development and to support process control for high volume manufacturing.

Sustainable Solutions for Copper Balancing
Making a second appearance at the conference, Dr. Ing. Agnieszka Franczak from Elsyca presented a sustainable solution for automated and optimized copper balancing. She described copper balancing as a crucial design specification: Consistent copper coverage in each layer of the PCB stackup avoids electrical and mechanical issues that can impede the performance of the circuit, and inconsistent copper coverage in different layers leads to mismatched thickness on each side of the board after the lamination process. With many examples, she illustrated problems that can occur without good copper balance. 

She recommended a workflow for achieving symmetry and optimizing copper balance during the circuit design stage, and using grids instead of solid planes wherever possible. In her experience with standard design tools, copper balancing operations have to be done manually; the operation is time-intensive and often gives inferior results.

In developing a dedicated PCB balance tool, she had discussed and collaborated with many designers and fabricators. As a test-case example, she discussed the results of an independent comparison between a standard design tool and Elsyca’s automated PCB copper balancing solution that uses a finite-element method to achieve optimal build-up thickness and distribution.

She went on to describe how the metal balancing fraction for a certain finite element in the balancing area is based on the active metal fraction of surrounding finite elements. A balancing element close to an active area with high active metal fractions will get a different balancing fraction than when it is closer to an active area with lower active metal fractions. As a result, balancing fractions will vary throughout the balancing area, influencing the thickness of the resulting metal layer in the surrounding active area. The system is capable of generating an optimal copper balance within seconds and can be integrated into existing CAD platforms.

Ink-Jet Strategies
The session on solder mask applications and surface finishes was moderated by EIPC technical director Tarja Rapala-Virtanen, and the first speaker she introduced was Hans Fritz, still involved at SAT as innovations manager, who described an ink-jet strategy for printed circuit boards. 

Having summarised the benefits of ink-jet for the application of solder masks, legends and flex coverlays, Fritz discussed the basic principles of ink-jet resolution, with some interesting observations on the coalescence of successive droplets. 

Compared with laser direct imaging, the resolution of ink-jet is relatively coarse, limiting the ultimate accuracy that can be achieved, Fritz has observed a capillary effect when printing a line as a raster image that can cause a positional error of about 10 microns unless successive lines are printed far enough apart to avoid the interaction of adjacent droplets. He commented that the physics sometimes helps and sometimes gets in the way, particularly regarding capillary and surface tension effects. His printing strategy involves generating several raster pictures at the native resolution of the print-head in order to overcome these physical effects. He described printing an initial “preparation” layer, fixed immediately by high-power UV, followed by a second layer to fill in areas between the conductors, and finally a layer which was allowed to flow slightly to yield a glossy surface. He showed some cross-sections, illustrating the ability to achieve different thicknesses in different areas as required, with uniform coverage over the conductors including the corners.

Declaring that he had made it his task to address the challenges of ink-jet printing and that he will actually retire “when inkjet works,” Fritz responded to a question from the floor asking whether he could retire tomorrow: “There are still plenty of projects to follow.”

REACH Regulations
REACH regulations are adopted to improve the protection of human health and the environment from the risks that can be posed by chemicals, but inevitably present challenges to material suppliers. Harald Kutzias, technical sales engineer with Taiyo America, described how to formulate an SVHC-free solder mask for high temperature applications. 

He summarised the factors to be considered when developing new solder mask products, which include performance criteria, processability, and regulatory requirements. Whereas in the past, performance criteria and processability were top of the formulator’s checklist with regulatory requirements, although important, ranked behind them, today all three factors have to be treated equally when developing a new product since regulatory requirements can limit the number and types of ingredients that can be used. 

He explained that the European Chemicals Agency (ECHA) manages the REACH regulations and reviews the hazards of chemical substances used in the EU. ECHA has established a list of items considered unfriendly to the environment and human health, known as the Substances of Very High Concern (SVHC) list. Items that appear on the SVHC list are subject to further evaluation, which could lead to restrictions on the use of the substance. Some of the items on the SVHC list have been used in solder mask formulations for many years.  

Because of possible restrictions, formulators are finding alternative substances for new solder mask products, and selecting the correct substances can be very challenging since the SVHC-free product must have equal or better performance than the original product. 

In the case of an SVHC-free solder mask for high temperature applications, the challenges include finding a suitable replacement for conventional photoinitiators, such as I-907. This involves evaluating over 30 photoinitiators and photoinitiator combinations. It is also necessary to choose a high temperature resin system compatible with the solder mask formulation and to determine the correct inorganic filler particle size and shape. Kutzias described how Taiyo America’s R&D team has gone about selecting and evaluating the formulation ingredients and optimised filler loadings and resin-to-photoinitiator ratios to achieve an SVHC-free product with excellent high temperature performance.Page 2 of 3

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