Advanced Build-up Materials for High Speed Transmission Application Open Access

Printed circuit boards such as multi-layer printed wiring boards and flexible printed wiring boards are used in a wide variety of electronic devices. With increasing demands for 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 using a semi-additive process are widely used in IC packaging substrates [2]. The manufacturing process of multilayer printed circuit boards by a semi-additive process using insulation build-up films which includes 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 the insulation layer and the plated copper layer by roughening the surface of the 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) through 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 to be converted 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. Therefore, 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.

Our insulation build-up materials for a semi-additive process (Ajinomoto Build-up Film, which is abbreviated to “ABF” in this paper) show low dielectric loss tangent, good insulation reliability between thin layers, and smooth resin surface after desmear. Additionally, the measurement results of transmission loss of a strip line substrates manufactured with low dielectric loss ABFs are described. Furthermore, the low dielectric molding film with low CTE and low Young's modulus is introduced to be available for Fan-out wafer level package (FOWLP) and panel level package (PLP).

Table 1 shows the characteristics of conventional ABFs called GX series. GX13, GX92 and GX-T31 have been released to the market in order, reducing these dielectric loss tangent and CTE. SEM images of GX series' surface after desmear shown in Fig.1 indicate that surface roughness of GX-T31 is lower than that of GX13. Furthermore, a new ABF with a thin copper transfer film using the specific phenomenon was developed that obtained the high adhesion between ABF and the smooth pure copper deposited by sputtering or vacuum evaporation [5–7]. This copper transfer film achieved a very flat surface of combined ABF, which was expected to reduce the transmission loss in high frequency range.

GX series include the epoxy resin and phenol hardener, which generate a highly polar secondary hydroxyl group after the curing (cross-linking) as shown in Fig. 2. Therefore, it is quite a challenging to develop GX series withlow dielectric loss tangent. To overcome this limitation, other curing systems have been applied to the low dielectric loss ABFs

Recently GZ series which include a cyanate ester resin as an epoxy hardener were developed. GZ series generate triazine ring and oxazolidone ring as shown in Fig. 2. Those groups is less polar than secondary hydroxyl group. In addition, GY series including the epoxy resin and phenolic ester hardener were developed as shown in Fig. 2. A secondary alcohol group is protected by an acyl transfer reaction after the curing of GY series.

Table 2 shows the characteristics of GZ series called GZ-41 and GY11 after curing at 190 degC for 90 minutes. GZ- 41 indicates that half of the dielectric loss tangent as that of GX-T31. Especially, GY11 shows the lowest dielectric loss tangent with smooth resin surface after desmear as shown in Fig. 1. The adhesion strength between these ABFs and plated copper is 0.5–0.6 kgf/cm and the insulation reliability is excellent. Recently, a new low dielectric loss ABF called GL was developed as showed in Table 2 and Fig.1. Although GY11 described in this paper shows the good electrical characteristics, relatively long time in desmear process is needed to remove the smear at the bottom of small vias drilled by CO₂ laser which is commonly used for via formation. On the other hand, GL series were developed to to remove the smear at small vias made with CO₂ laser in a shorter desmear time, showing as low dielectric loss as that of GY11.

The dielectric constant and loss of ABFs were measured over a temperature range of −40 degC to 150 degC with frequency ranging from 10 GHz to 100 GHz. Dielectric constant of all ABFs measured was almost the same at various frequencies and temperatures (Data not shown in this paper). Dielectric loss tangent also showed stable over a wide frequency range and temperature range as shown in Fig. 3. This temperature independence of dielectric loss is mainly due to the high glass transition temperature (Tg) of ABFs being stable over a wide temperature range [8].

Actually, transmission loss of a strip type line substrate were measured using each ABF. Fig. 4a shows the cross sectional image of the substrate and Fig. 4b shows the measurement results of transmission loss at various frequencies. GY11 showed the lowest transmission loss in all ABFs measured, indicating that the lower dielectric loss and smoother surface roughness of ABF are effective for reducing the transmission loss.

Additionally, Cu surface treatment is also important to reduce transmission loss. Generally, Cu surface of pattern is roughened to achieve good adhesion to the resin. However, if the Cu surface roughness become higher, the transmission loss increase more, especially at high frequencies due to the skin effect [9]. Therefore, Cu surface roughness is one of most important factors related to transmission loss for high frequencies applications. The impact of roughness of Cu with GL material were investigated recently. Fig. 5 shows cross-section images of a Cu line with different surface treatments and results of examinations. From the results, the smoother Cu surface showed the lower transmission loss at various frequencies. The result indicated that decrease of Cu surface roughness is effective to prevent transmission loss, and then it would be necessary for build-up films for high speed applications to adhere Cu well even though Cu surface become quite low. From the measurement results of adhesion strength between low loss types of ABF (GY11, GL) and very smooth surface of Cu, the low loss materials showed good adhesion of more than 0.5 kgf/cm after curing.

FOWLP has advantages in cost, thickness, and integration density over conventional packages such as Fan-in WLP and FC-BGA. Recently, some companies are trying to build high speed packages such as antenna modules for millimeter wave 5G automotive applications [10].

ABF is applicable to mold for fan-out WLP/PLP due to its good resin flow, excellent insulating reliability, thickness uniformity, and fine line and space formation by SAP. In order to reduce the warpage after the one-side resin curing for manufacturing FOWLP, a molding film called LE series was developed. LE series use the special polymer including flexible segment for relaxation on internal stress. This polymer also includes hard segment and reactive segment with epoxy resin for the high temperature resistance and good insulation reliability.

After the WLP/PLP molding process using LE series, molding material showed the low warpage as shown in Fig. 6. This modest warpage is coming from the internal stress relaxation induced by low CTE and low Young's modulus of LE characteristics described in Table 3. In addition, low dielectric loss of LE series seems promising for high speed FOWLP.

For high speed transmission application, low dielectric loss GZ-41, GY11, and GL were developed. These materials showed high reliability and strong adhesion with copper layer in spite of their smooth surface after desmear. These dielectric properties were stable over a wide range oftemperature and frequency range. In addition, these low dielectric loss ABFs were considered to reduce the transmission loss of strip line substrates. And, how effective the decrease of Cu surface roughness was measured for reducing transmission loss. Furthermore, the low dielectric molding film LE was developed. LE showed low warpage after the one-side resin curing due to the low CTE and low Young's modulus. The experimental results confirm that these ABFs are suitable for manufacturing the printed circuit boards and IC packages of next generation's high frequency electronic devices.

We would like to thank Research Center for Three Dimensioned Semiconductors, Professor Tomokage and his laboratory at Fukuoka University for the technical support of building up the strip type line board.