Oxide Alternative Process for High Frequency Bonding Applications

Neal Wood, Patrick Brooks, Thomas Thomas, Thomas Huelsmann, Tatjana Koenigsmann, Andry
Liong, Wonjin Cho, Carrick Chan
Atotech Deutschland GmbH, Berlin

The demand for smaller, faster and smarter electronic devices that can communicate to each other via wireless networks is stretching current communication systems to their limits. This rapid paced evolution is the driving force for the development of the next generation of network systems, the so-called 5G, to give the increased speed of communication and data capacity required.

One of the key factors, which will influence the development in 5G, is the range of frequencies at which data is transferred. Current 4G systems operate in relatively low frequencies of <6GHz. With the introduction of 5G systems, the requirement will be to run at much higher frequency band width; typical range will be from 6 up to >100GHz to enable faster mobile broadband, with low latency (the time taken for devices to respond to each other or a signal from another device) and the “internet of things” where compatible devices “talk to each other”.

With this transition to higher frequencies, the path of the electrical signal moves towards the edge of the copper traces into the so-called “skin” or the extreme outer edges of the copper trace. If this “skin” has been heavily roughened or etched, such as is the case in conventional multi-layer bonding enhancement processes, then there is an unacceptably high loss of the electrical signal.

To overcome this high signal loss material suppliers have been developing reliable high-speed dielectrics with low dissipation factors and dielectric constants. In combination with this, the contribution to signal loss from the bonding enhancement chemistry is under great scrutiny. Here, the challenge is to provide the most functionally reliable bonding process with the minimal surface roughening; but as the widely used Oxide Replacement bonding enhancement systems rely heavily upon surface roughening to give good bond strength this creates something of a challenge.

This paper highlights the challenges, developments and modifications made with conventional Sulphuric-Peroxide based Oxide Replacement bonding enhancement system to meet the low signal loss requirements for High Frequency applications, whilst maintaining the highest functional performance and bonding integrity needed for manufacture of reliable multi-layer PCB’s.

There is a high demand for bonding enhancement systems compatible with the needs of high frequency applications. The challenge in the development of Bonding Enhancement chemistry is to provide a system that does wholly not rely on high surface roughening to provide bond strength and good thermal reliability. Essential the ultimate goal is for a non-etching, non-roughening system that would purely rely upon chemical bonding between the copper traces and polymers in the dielectric system.

Currently no such system is known and while the search for such a process continues, modifications to the current Sulphuric-Peroxide based Oxide Alternative system widely used in multi-layer circuit manufacture can help meet the near-term requirements for high frequency production.

By making changes to the etch modifier compounds used in Oxide Alternatives, it is possible to somewhat reduce the surface roughening whilst maintaining the desired bond strength and thermal properties of the board. This is achieved by a synergy of mechanical and chemical bonding, which can also be further enhanced by the application of an adhesive post dip coating based upon an organo-Silane chemistry.