A Comparison of Adhesion Behavior of Urea-Formaldehyde Resins with Melamine-Urea-Formaldehyde Resins in Bonding Wood

This paper reports a comparison of adhesion behavior of urea-formaldehyde (UF) with those of melamine-urea-formaldehyde (MU) resins in bonding wood by analyzing the results published in literatures. For this purpose, the adhesion behavior of UF resins prepared by blending low-viscosity resin (LVR) with high-viscosity resin (HVR) at five different blending and two formaldehyde/urea (F/U) molar ratios (1.0 and 1.2) was compared with those of two MUF resins synthesized by either simultaneous reaction (MUF-A resins) or multi-step reaction (MUF-B resins) with three melamine contents (5, 10, and 20 wt%). As the blending (LVR:HVR) ratio increased from 100:0 to 0:100, the viscosity and molar mass (Mw and Mn) of the blended UF resins increased while the gelation time decreased. The interphase features such as maximum storage modulus (E′max), resin penetration depth, and bond-line thickness of the UF resins increased to a maximum and then decreased as the blending ratio increased. In addition, both MUF-A and MUF-B resins also showed an increase in the Mw and Mn as the melamine content increased from 5% to 20%. However, the E′max, resin penetration depth, and bond-line thickness of the MUF resins decreased as the molar mass or melamine content increased. These results indicated that the adhesion of UF resins heavily depends on the interphase features while that of the MUF resins highly depends on the cohesion of the resins.

Semiconductor Chip Electrical Interconnection and Bonding by Nano-Locking with Ultra-Fine Bond-Line Thickness

The potential of an innovation for establishing a simultaneous mechanical, thermal, and electrical connection between two metallic surfaces without requiring a prior time-consuming and expensive surface nanoscopic planarization and without requiring any intermediate conductive material has been explored. The method takes advantage of the intrinsic nanoscopic surface roughness on the interconnecting surfaces: the two surfaces are locked together for electrical interconnection and bonding with a conventional die bonder, and the connection is stabilized by a dielectric adhesive filled into nanoscale valleys on the interconnecting surfaces. This “nano-locking” (NL) method for chip interconnection and bonding is demonstrated by its application for the attachment of high-power GaN-based semiconductor dies to its device substrate. The bond-line thickness of the present NL method achieved is under 100 nm and several hundred times thinner than those achieved using mainstream bonding methods, resulting in a lower overall device thermal resistance and reduced electrical resistance, and thus an improved overall device performance and reliability. Different bond-line thickness strongly influences the overall contact area between the bonding surfaces, and in turn results in different contact resistance of the packaged devices enabled by the NL method and therefore changes the device performance and reliability. The present work opens a new direction for scalable, reliable, and simple nanoscale off-chip electrical interconnection and bonding for nano- and micro-electrical devices. Besides, the present method applies to the bonding of any surfaces with intrinsic or engineered surface nanoscopic structures as well.

Electrical Interconnection and Bonding by Nano-Locking

The growing demand for increased chip performance and stable reliability calls for the development of novel off-chip interconnection and bonding methods that can process good electrical, thermal, and mechanical performance simultaneously as well as superior reliability. A chip bonding method with the concept of “nano-locking” (NL) is proposed: the two surfaces are locked together for electrical interconnection, and the connection is stabilized by a dielectric adhesive filled into nanoscale valleys on the interconnecting surfaces. The general applicability of this new method was investigated by applying the method to the die-substrate bonding of two different packages from two different manufacturers. Electrical, optical, and thermal performances as well as reliability tests were carried out. The surface morphology of the bonding package substrates plays an important role in determining the contact resistance at the bonding interfaces. It was shown that samples with different roughness height distribution on the metallic surfaces formed a different total number of contacts and the contact area between the two bonding surfaces under the same bond-line thickness (BLT): a larger number of contact area resulted in a reduced electrical resistance, and thus an improved overall device performance and reliability.

Effect of Bond-Line Thickness on Fatigue Crack Growth of Structural Acrylic Adhesive Joints

With an increasing demand for adhesives, the durability of joints has become highly important. The fatigue resistance of adhesives has been investigated mainly for epoxies, but in recent years many other resins have been adopted for structural adhesives. Therefore, understanding the fatigue characteristics of these resins is also important. In this study, the cyclic fatigue behavior of a two-part acrylic-based adhesive used for structural bonding was investigated using a fracture-mechanics approach. Fatigue tests for mode I loading were conducted under displacement control using double cantilever beam specimens with varying bond-line thicknesses. When the fatigue crack growth rate per cycle, da/dN, reached 10−5 mm/cycle, the fatigue toughness reduced to 1/10 of the critical fracture energy. In addition, significant changes in the characteristics of fatigue crack growth were observed varying the bond-line thickness and loading conditions. However, the predominance of the adhesive thickness on the fatigue crack growth resistance was confirmed regardless of the initial loading conditions. The thicker the adhesive bond line, the greater the fatigue toughness.

Establishing Robust Control for Epoxy Open Time

The Open time is called the time it takes for this chemical transition from liquid to solid. The epoxy moves into a gel state from the liquid state as it recovers, until it enters a solid-state. This article will address the development of a new, regulatory-compliant in die attach epoxy that establishes robust Epoxy Open Time control to improve the performance of product reliability with the following quality output response characteristic in Die Attach was consider; Epoxy Coverage, epoxy voids, Bond Line Thickness (BLT), and Die Shear Test (DST) strength response.

Bond Line Thickness Characterization for QFN Package Robustness

The paper focused on the evaluation of quad-flat no-leads (QFN) semiconductor package with small silicon die on different machine platforms to achieve a higher bond line thickness (BLT) of greater than 30 µm. The characterization or evaluation was narrowed down into two main diebonding machines with the objective of attaining a higher BLT for small die. High BLT capability is desired to generate clearance for the shrinkage of the glue, henceforth mitigating the glue voids. Diebond Machine 2 was able to achieve the target BLT with 30.89 µm average compared to 18.25 µm for Machine 1. Moreover, the target BLT range could only be achieved in Machine 2 only. For future works, the machine and configuration could be used for devices with comparable requirement.