Honeywell

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Advanced Build-up Materials for High Speed Transsmission Application
Keywords: Insulation build-up material, Semi-additive process, Low transmission loss
Printed circuit boards such as multi-layer printed wiring boards and flexible printed wiring boards are used for a wide variety of electronic devices. Along with increasing demands of downsizing electronic devices with high functionality, packaging substrates installed with semiconductors in such devices are strongly required to be miniaturized with high density of circuit wirings [1]. Accordingly, insulation materials are also required to show low coefficient of thermal expansion (CTE), to produce fine line formation, and to show good insulation reliability between thin layers. Printed circuit boards built in a semi-additive process are widely used for IC packaging substrates [2]. The manufacturing process of multilayer printed circuit boards by a semi-additive process using insulation build-up films is comprising of the lamination, the curing of insulating film, the formation of laser vias, and desmear process by alkaline permanganate solution to form micro anchors on the surface of insulation layers. Then, electroless copper plating is performed as a seed layer for thick electrolytic copper plating This process provides high peel strength between an insulation layer and a plated copper layer by roughening the surface of insulating layer. However, this anchor effect is a disadvantage for fine line formation and high speed signal transmission. A relatively long etching time to remove a seed layer of anchor parts at the flash etching step (differential etching) of semi-additive process makes noticeable dissolution of fine copper lines. Furthermore, high speed transmission of a large amount of digital data is strongly required for high-end electronic devices such as 5G communication terminals, millimeter wave radar, and networking servers [3]. When electrical signals flow through circuit wirings, electrical signals are attenuated because the polarization of insulating material causes high consumption of the signals with conversion into heat. In general, a high frequency electrical signal is used for high speed transmission. The attenuation of electrical signals is larger at higher frequencies, higher dielectric constant and loss tangent due to increasing the insulator loss [4]. In addition, the skin effect on the conductor surface significantly affects the transmission loss, especially in a high frequency range. Accordingly, materials having lower dielectric loss tangent and lower dielectric constant as well as the smooth resin surface after desmear are required for high speed transmission applications. We developed our own insulation build-up materials named Ajinomoto Build-up Film (ABF) for semi additive process. GX series are conventional ABFs which have been already released to the market. GX series include the epoxy resin and phenol hardener mainly. With GX series, the secondary hydroxyl group appears after curing reaction. Therefore, it is challenging for GX series to meet the demand of high frequency electric devices because the hydroxyl group has high polarization and cause the transmission loss. To reduce highly polarized hydroxyl alcohols after curing, we developed GZ and GY series which include cyanate ester hardener and phenolic ester hardener respectively. Regarding GZ reaction system, triazine ring and oxazolidone ring are generated after the curing which have lower polarization rather than the hydroxyl group. Similarly, a secondary hydroxyl group is capped by acyl transfer reaction with GY series after curing. Especially, GY material shows low dielectric loss tangent and low surface roughness after demear which is advantage for fine line formation and transmission loss regarding the skin effect. And then, high peel strength between the insulation film and plated Cu is able to be obtained despite of smooth resin surface after desmear. However, GY material needs relatively long time in desmear process to remove the resin residue at the bottom of vias drilled by CO2 laser which is commonly used. Recently, we developed the new low dielectric loss ABF called GL. GL also shows low transmission loss tangent and smooth resin surface after desmear. However, compared to GY, GL is more suitable for conventional semi additive process because its resin residue drilled by CO2 laser is easily removed, and only to shorten desmear time. We measured the frequency and temperature dependences of transmission loss tangent for GL. In the temperature range from -40 degC to 150 degC with frequency range from 10GHz to 100GHz, the value of loss tangent gradually increased however it was stably low. Here, we are going to report current evaluation results of GL which indicates advantageous features for high speed transmission applications, and add some materials information for next electric devices.
Shiro Tatsumi,
AJINOMOTO CO., INC.
KAWASAKI, KANAGAWA
JAPAN


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