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Study on New Process to Improve Bonding Strength of the Inner Layer

Author: HuangGuiPing
Tutor: ZhangWeiDe;LuoZhong
School: South China University of Technology
Course: Applied Chemistry
Keywords: printed circuit boards oxide replacement lead-free corrosion inhibitor peeling strength thermal properties
Type: Master's thesis
Year: 2010
Downloads: 71
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In order to meet the requirements of high temperature for lead-free soldering, PCB substrate materials have been developed from the general performance to lead-free compatible materials. The resin system of lead-free compatible substrate is mainly epoxy with multi-functional groups, while curing agent is mainly phenolic resin. High-density phenolic molecule with benzene ring structure increases the molecular rigidity, and a large number of hydroxyl groups in the epoxy can make the ring-opening reaction with epoxy and the cross linking density is increased after curing. Phenolic resin can improve the lead-free heat resistance and chemical resistance of substrate. As lead-free compatible materials mainly contain aromatic structure, the resin exhibits low adhesion strength.Oxide replacement process is an inner layer bonding treatment technology for manufacture of multi-layer printed circuit board. This process is short and easy to implement. Oxide replacement working solution, which mainly contains benzotriazole as an organic additive, together with sulfuric acid, hydrogen peroxide and chloride ions. When the copper is treated by oxide replacement solution, micro-rough structure will be created on the copper surface and corrosion film will cover on the copper surface as well. After being treated by oxide replacement process, the copper will be submitted to be pressed with resin under high temperature for good adhesion between copper and resin. Peeling strength is an important indicator of bonding between copper foil and resin. However, the development of lead-free materials, resulting in low peeling strength between copper and the resin. In order to minimize delamination issue caused by low peeling strength, this study will develop a new oxide replacement recipe, with etching depth of 1.2-1.3μm, to increase the peeling strength between copper foil and lead free compatible resin with target 15% or more. Regarding to the thermal performance, the test specimens should pass at least 6 cycles of solder dipping and lead-free reflow test without delamination, and the time to delamination (T260) should be more than 30 minutes.In this paper, benzotriazole is used as main corrosion inhibitor. According to the synergy mechanism of corrosion inhibitor, four types of organic inhibitors, which are uracil (code X1), azole (code X2), organic matter containing furfural (code X3) and heterocyclic substances (code X4) are added to the oxide replacement solution. Etching depth is controlled between 1.2μm and 1.3μm, the results show that peeling strength can be increased 6.3%, 23.2%, 2.8%, 8.8% respectively after adding X1, X2, X3, X4. Among these four additives, only azole inhibitor X2 can reach the peeling strength target. Thermal properties tests show that the specimens treated by new recipe with adding X2 can meet thermal properties requirements.In order to further analyze the reaction mechanism, scanning electron microscopy, atomic force microscopy, focused ion beam and X-ray photoelectron spectroscopy are used to analyze the copper surface. The results show that the copper surface has been formed micro-honeycomb structure after being treated in oxide replacement solution which contains benzotriazole. The mechanism is corrosion reaction between the boundary of copper crystal and inhibition film forms on the copper surface. The treated copper foil reacts with resin under high temperature, and the resin can fully penetrate the micro-honeycomb structure of copper foil. The rough structure and inhibition film at the copper surface determine the peel strength values. The four corrosion inhibitors are added to the working solution, but they do not change the surface roughness of copper. X-ray photoelectron spectroscopy indicates peeling strength is related to content of nitrogen content in the copper surface. The copper foil with highest peeling strength is detected with highest nitrogen content.Orthogonal experiments are made to test the influence of four factors including X2, sulfuric acid, hydrogen peroxide and temperature on etching rate and peeling strength. The results show that X2 has met the requirements to improve peeling strength when the concentration is more than 5g/L. The concentration of X2 sulfuric acid does not affect etching rate and peeling strength obviously within the setting parameter. Hydrogen peroxide and temperature influence etching rate and peeling strength significantly. With the same immersion time, the higher etching rate can achieve higher peeling strength. The optimal parameter is defined from the orthogonal experiments, as well as the influence factor of etching rate and peeling strength.With etching rate of 1.2-1.3μm/min, the optimal parameters of the oxide replacement bath are obtained.This paper has achieved the expected objective, and provided good solutions for the future development of new oxide replacement technology.

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CLC: > Industrial Technology > Radio electronics, telecommunications technology > Microelectronics, integrated circuit (IC) > Printed circuit
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