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The Use Of Potassium Silicate To Promote Sub-oxide Bonding To Solder Glass

IP.com Disclosure Number: IPCOM000004658D
Original Publication Date: 2001-Mar-20
Included in the Prior Art Database: 2001-Mar-20
Document File: 5 page(s) / 336K

Publishing Venue

Motorola

Related People

Alan James: AUTHOR

Abstract

One vacuum seal system for Field Emission Devices (FED) involves the use of vitreous lead solder glass (frit). Failure analysis of a frit seal problem showed that the decreased seal strength was due to a poor frit bond to the cathode. This prompted an investigation into finding an interim material that would assist in that bond yet would not in itself degrade the vacuum seal integrity. The material used was potassium silicate.

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The Use Of Potassium Silicate To Promote Sub-oxide Bonding To Solder Glass

Alan James

ABSTRACT

One vacuum seal system for Field Emission Devices (FED) involves the use of vitreous lead solder glass (frit). Failure analysis of a frit seal problem showed that the decreased seal strength was due to a poor frit bond to the cathode. This prompted an investigation into finding an interim material that would assist in that bond yet would not in itself degrade the vacuum seal integrity. The material used was potassium silicate.

INTRODUCTION

Starting in March of 1998 meetings were held to discuss the issue of FED packages with poor seal strength. During the kick off meeting Reg Parker expressed his concern about the variations in cathode oxide thickness which could be seen as color variations in the cathode seal area. Subsequently seal pull test data was presented to support the hypothesis that oxide variations were directly involved in seal strength. Although the failure mechanism causing the degradation had not yet been identified, this analysis led to the testing of a reinforced cathode oxide structure known as the "SiO2 seal ring". After inspecting the failed seals I felt that potassium silicate could be used as an interim technique to aid in bonding the frit to the cathode.

The accepted use of silicic acid, potassium salt (AKA. potassium silicate, kasil, K2) has proven invaluable in the vacuum tube industry. Kasil has been widely used as an adhesive for both the phosphor and the reflective aluminum backer in anode manufacturing, as an adhesive for conductive carbon coatings and as part of a glaze system to prevent charging of glass and alumina substrates that are subjected to high electron fluxes. The adhesive mechanism is through a process called "cementation" in which dehydration acts as a catalyst to turn the potassium silicate into an amorphous glassy material. When properly outgassed kasil is vacuum packaging compatible. It was my contention that kasil could be used as an interface to assist the subsurface oxide bond needed for a hermetic frit seal.

TEST SETUP

Kasil was mixed, filtered and applied to the components making up the seal system. After the application, the anode and frame were air fired to 450oC for one hour. The cathode was vacuum fired at 450oC for one hour. The effect of the first high temperature firing of kasil is to liberate water vapor and either firing in air or vacuum will suffice. Final component processing was accomplished to internal processing standards.

In all, four packages were sealed then subjected to a pull test per our in-house process specification. The test setup consists of a Chatillon force gauge mounted on a test stand with a hand crank. The package is secured to the test stand utilizing screw adjustable clamps that hold the bottom glass panel. The force gage lip is slipped under the top glass panel. Turning the hand crank drops the package away from the force gauge, pulling the top panel in turn.

RESULTS

One package was...