2017 IEEE 67th Electronic Components and Technology Conference
Development of mechanical locking micro-anchor structures for aQFN package application Yu-Lung Huang1, Mano Ajayan1, Bao-Hui Chang Chien2, Wei-Chih Lin1* 1
Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-sen University 2 Advanced Semiconductor Engineering, Inc. 1 No.70, Lienhai Rd., Kaohsiung 80424, Taiwan 2 No.26, Chin 3rd Rd., Nantze Export Processing Zone, Kaohsiung 811, Taiwan email:
[email protected] Abstract—Due to a weak adhesion, the delamination occurred frequently in the interface between the epoxy compounds and lead that results in a performance failure normally. Several treatments, such as plasma and chemical modifications, have been developed and applied to enhance the bonding force in the interface of both materials. In this study, an anchor shape of the lead, as a mechanical locking microstructure, was developed and fabricated on the lead to improve the bonding of packaged epoxy molding compound. The fabrication technique was composed of three mainly steps which are the photolithography, chemical wet etching and electroplating process. The parameters of each step were studied and obtained for the fabrication of copper micro-anchor structure. Furthermore, force-displacement curves corresponding to the separation were obtained from the finite element simulations using a linear elastic model. The simulation result showed that the pull-off force was raised by 69 percent.
Figure 1. SEM image of the delamination occured between lead and epoxy molding compound in aQFN package
There have been a few surface treatment methods committed to improving the adhesion strength between lead and EMC including oxidation growth, plasma treatment, coupling agent [6-15]. The most methods aimed to modify the surface property such as roughness, contact angle or to form the chemical covalent bonds between copper substrates and EMC to improve the adhesion of those two material. The oxidation growth which could make the mechanical interlocking effect by acicular CuO layer was the widely method to improve adhesion strength [8]. Takano, E., et al. suggested that the oxide layer thickness lower than 20nm could have the higher adhesion strength [9]. Sung Yi et al. indicated that plasma treatments of leadframe not only could clean surface due to the removal of organic contaminants, but also could rough surface and enhance chemical compatibility with EMC [11]. Song et al. improved the adhesion of EMC to the copper by eight times with the use of azole and triazole compounds as coupling agent [14]. But the prolongs surface treatment time would decrease the adhesion strength. To make more stronger polymer-metal joining and reach the other demand, adhesion method could combine with the other joining method such as spot weld, rivet, clinch, thread fastener to form the hybrid joining [16]. Nguyen, L. T. and M. Michael combined the origin adhesion and mechanical anchoring between the EMC and die pad with holes or slots to form the hybrid joining [17]. That die pad design also reduced the relative motion from thermal mismatch and the risk of moisture accumulation under die pad. Additionally, the anchor-shaped lead had been utilized in aQFN package to improve bonding by the
Keywords-Delamination; Micro-anchor structure; Mechanical locking effect; Average stress approach
I.
INTRODUCTION
The aQFN package, the advanced quad flat no-lead, had the better performance in thermal and electrical than QFN and TFBGA due to the smaller profile, shorter interconnects. It also could provide the similar I/O number compared with that of a BGA-type chip scale package within lower cost [1]. Therefore, the aQFN package had been widely used in portable telecommunication devices such as mobile device, IrDa, blue-tooth, RFID. Because of the thermal expansion mismatch, the moisture absorption, geometrical discontinuities and the degradation of interfacial adhesion, the QFN package had a history of the issue about interface delamination between lead and epoxy molding compound(EMC) [2-4]. The aQFN package also had the similar issue. The Fig.1 showed that the delamination between lead and EMC occurred inside the aQFN package. Moreover, Cracking in homogeneous materials such as EMC and interconnection layers were often triggered by interface delamination [5]. In order to make the aQFN package more reliable, the purpose of this study was the improvement of the bonding between lead and EMC by the mechanical locking effect of the specific lead structure instead by the surface treatment. 2377-5726/17 $31.00 © 2017 IEEE DOI 10.1109/ECTC.2017.80
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B. Chemical wet etching process Fig 3 (b) showed the wet chemical etching procedure and the etchant was used to remove the unpatented partial copper material from the copper substrate. The alkaline copper chloride dehydrate (Cu:66g, NH4Cl:108g, pH:8, H2O:400ml) served as the etchants and was utilized to fabricate lead microstructures of aQFN [18]. This process was followed by the etching equation:
hybrid joining combining mechanical locking effect and adhesion. Because of the fabrication requirement of the anchor-shaped lead, fabricating this anchor-shaped lead would limit the design rule such as the minimum pitch was 650μm and the minimum line space was 200μm. This limitation of design rule could cause fabrication cost rise about 30%. To overcome the above reasons, this study had developed the micro-anchor structure composed the common lead structure of aQFN and copper plating layer to replace the previous anchor-shaped lead, as shown in Fig.2 (b). The structure profile of the micro-anchor structure developed was presented by using SEM. The lead pull-off test was conducted by FEM simulation to quantify the improvement of bonding by replacing the common lead microstructure to the micro-anchor structure developed.
(1) As shown in Fig. 3(b), the common lead microstructure array was formed after the chemical wet etching process. The etching time of 7 minutes were applied in the wet etching process. Then, the acetone was utilised to remove the photoresist patterns as illustrated in Fig.3 (c). C. Copper electroplating The photolithography process was repeated to fabricate a thicker photoresist layer as the electroplating barrier layer for forming electroplated copper micro-cap structure on the top of the lead microstructures by using electroplating process. For this purpose, the coating parameter of the spin coater was down to 1200 rpm for 10 seconds. The parameters of soft bake were 150ć and 70 seconds. The 365 nm ultraviolet exposures system was used for 70 seconds to transfer lead patterns from photomask to thick photoresist layer. Then, the same developer solution was utilised to reveal the top of lead in the developing process as illustrated in Fig. 3 (d).
Figure 2. Illustration of (a) the common lead microstructure and (b) mechanical locking micro-anchor structure of aQFN package
II. METHODOLOGY The fabrication of anchor shaped lead sample was divided into three main processes which are the photolithography, chemical wet etching and electroplating steps as shown in Fig. 3. A. Photolithography proces In this step, the AZ4620 photoresist patterns were fabricated on the C194 substrate by using photolithography as presented in Fig. 3 (a). The utilised photo mask contained three geometries of patterns as the following sizes and geometry as a circle in the diameter of 300 μm, square and rectangle in the width form of 300 μm and 20 × 175 μm, respectively. The approximate 40 μm thick of AZ4620 photoresist layer was uniform coated by using spin coater with 1200 rpm for 10 seconds then 2500 rpm for 20 seconds and baked at temperature of 150ć in 40 seconds for soft bake process. The 365 nm ultraviolet exposures system was used for 45 seconds to transfer lead patterns from photomask to photoresist layer. After the soft bake, the sample soaked in the developer solution of AZ400k and H2O in a volume ratio of 1:3 for 3 minutes to remove the rest photoresist. Final, the hard bake was presented by the condition of 150ć and 3 minutes.
Figure.3 The fabrication process of micro-anchor structure
As presented in Fig.3 (e), the electroplating was employed to fabricate electroplated copper micro-cap
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structure on the lead. The sample was immersed in the electroplating solution (CuSO4∙5H2O:92g, H2SO4: 15.7 ml, H2O: 400 ml) with the current density about 4.6A⁄dm2 to make the copper plating layer be electroplated on top of the lead structure to form the electroplated copper microcap structure[19]. The electroplating time was 55 minutes. After the electroplating process, the photoresist was removed by acetone. The fabrication of micro-anchor structure was completed as demonstrated on Fig.3 (f). The anchor part of the structure developed could be treated as a hook to mechanically lock the EMC. The SEM equipment (JEOL JSM 6380) was used to character the profile and the dimensions of this micro-anchor structure.
substrates was that the normal and shear adhesion strength are 29.52Mpa and 9.62Mpa sequentially obtained from shear button test. The assumption of this model was that (a)the material property of the lead microstructure and electroplated copper micro-cap structure were the same and (b)the junction between lead structure and electroplated copper micro-cap structure was perfect. The average stress approach was employed as the delamination initiation criterion of this model. This failure criterion had been successfully employed for predicting delamination initiation in the other adhesion test such as shear button test and leadframe pull-off test [20-22]. Final, the loaddisplacement curve which the common lead microstructure and the micro-anchor structure were pulled off from EMC was plotted for determining the pull-off force.
D. FEM Simulations of lead pull-off test The pull-off force of the leadframe pull-off test was used to evaluate the bonding between EMC and leadframe in plastic encapsulated IC package. The micro-anchor structure developed was used as the lead of aQFN package and evaluated the bonding between EMC by lead pull-off test. The quantification of bonding improvement was conducted by comparing the pull-off force which the common lead microstructure and the micro-anchor structure were pulled off from EMC. The COMSOL Multiphysics 5.2a was applied to simulated the lead pulloff test. The boundary condition of simulation model for the lead pull-off test which the common lead microstructure was pulled off from EMC was shown in Fig.4.
TABLE I.
EMC C194
MATERIAL PROPERTY OF THE EMC AND C194 COPPER SUBSTRATE.
E[GPa]
Poisson’s ratio
19 115
0.3 0.33
υ[
]
1950 8860
E. Average Stress Approach The prediction of delamination initiation between the two materials by average stress approach depended on the normal and shear stress of the interface. Therefore, both the shear and the normal stresses be taken into Eq. (2) to account the value of K. In average stress approach, the Eq. (2) as bellow was the critical equation for predicting delamination. By the value of K, it can be used to consider whether the stress of interface was so large enough or not as to make delamination initiation occur. Therefore, the delamination initiation due to any combination of stresses was thus expected to occur when the value of K was equal to or larger than unity [21]. (2) (3) (4) In the Eq. (2), Z and S were the interface bond strengths in tension and shear, and are the average normal and shear stresses, respectively. Eq. (3) and Eq. (4) showed calculation of the average normal and shear stresses over the averaging distance from the free edge under the critical pull-off force. The (x) and (x) were the normal and shear stresses along the interface. Yi, S., et al. had been built the failure criterion by average stress approach for leadframe pull-off test to determine the adhesion strength and suggest that the averaging distance was equivalent to the lead frame thickness or 1/8 of its
Figure.4 The boundary condition of simulation model for the lead pulloff test
The bottom of the common lead microstructure was constrained in the vertical direction and an upward pulling force was applied to the top end of EMC. To reduce the calculation time, the geometry of this model was simplified as a quarter of origin one. The Sumitomo EME G700 was chosen as EMC. The material property of EMC and C194 shows in the Table. Ⅰ respectively. The interface adhesion strength between EMC and bare copper
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length embedded in EMC [20]. Therefore, in this model the 1/8 of lead length embedded in EMC was chosen as the averaging distance. III. RESULT AND DISCUSSION The following characteristic such as the structure and pattern of lead, were considered in aQFN package. A. Photolithographs Fig.5 demonstrates the patterns of photoresist, including circle, square and rectangle, were coated on the C194 copper substrate. Table. Ⅱ shows the basic information of pattern of lead such as the area of lead and space between each pattern.
(b) Figure. 6 (a) Top view photo image of the etched copper substrate with three geometry patterns. (b) SEM image shows the sidewall and depth of common lead microstructure.
Figure. 7 The micrograph image of patterned photoresist layer for the electroplating process.
Figure.5 The photoresist patterns including circle, square and rectangle.
TABLE II.
Area(mm2) Space(μm)
C. Copper electroplating Firstly, photoresist was coated on the substrate as the electroplating barrier layer as shown in the Fig.7. Secondly, the electroplated copper micro-cap structure grows up on the lead as increasing the time of electroplating process. Finally, the photoresist was removed from the substrate by using acetone. The microanchor structure was formed as shown in Fig.8 (a-c). The enlarge electroplating cooper micro-cap structure growing up on top of the lead microstructure was formed after the 55 minutes electroplating process so as to composed the micro-anchor structure. Fig.9 showed the array view of micro anchor structure of circle, rectangle and square. The radius of the enlarge electroplating cooper micro-cap structure of circle was larger 22.73 μm than the one of lead microstructure as shown in Fig.8 (a).
MATERIAL PROPERTY OF THE EMC AND C194 COPPER SUBSTRATE.
Circle 0.89 57.35
Square 1.085 41
Rectangle 4.653 44.86
B. Wet chemical etching After the wet chemical etching, the common lead microstructureġ was formed. Fig.6 (a) showed the top view photo and the pattern of the common lead microstructure. Fig.6 (b) demonstrate the height of lead 28.14 μm, was fabricated by using wet chemical etching with average etching rate was 5.45 μm per min. The average height of lead was investigated by surface profiler tool. The average height of lead structure was 27.6 μm.
(a) (a)
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(a)
(b)
(b)
(c) (c)
Figure. 8 The SEM image of the fabricated (a)circle, (b)square and (c)rectangular micro-anchor structures.
Figure. 10 (a) FEM simulation of the pull-off strength utilized common lead microstructure(non-anchor) and (b)micro-anchor(anchor) structure (c) Load-displacement curve calculated for the EMC separatedġfrom the common lead microstructure and micro-anchor structure.
when the pulling force was approximate 2.09N. The delamination initiation between EMC and micro-anchor structure occurs when pulling force was 3.53N. However, in the curve of anchor in Fig.10 (c), the pulling force wouldn’t decrease to zero instead to 0.72N and increase slowly. It means that the EMC was still well locked in micro-anchor structure after delamination occurs. That pulling force slowly increasing resulted from the locking force between EMC and micro-anchor structure until the cohesive failure of EMC and C194 copper substrate occurs. Final, the pull-off force which the micro-anchor structure was pulled from EMC was higher 69% than the one which the common lead microstructure was pulled from EMC. Therefore, the mechanical locking effect could effective improve bonding.
Figure.9 The SEM image of array view of micro anchor structure of circle, rectangle and square
D.
FEM Simulation of lead pull-off test The profile and size of the circle micro-anchor structure and common lead microstructure presented above were inputted into the simulation model to calculate the pull-off force which the common lead and the microanchor structure were pulled off from EMC. Fig.10 (a) and (b) reveal the FEM simulation of Von Mises stress distribution of EMC and common lead and micro-anchor structure during the delamination initiation occur, respectively. In Fig.10 (a) showed that the maximum stress of model mainly distributed in the lead microstructure. In the other model, the maximum stress mainly distributed at the electroplating bonding edge of micro-anchor structure as shown in Fig. 10 (b). Fig. 10 (c) presented that load-displacement curve calculated for the EMC separated from the common lead microstructure and micro-anchor structure. Two calculated separation forces are shown in figure. 10 (c). The EMC was fully separated from the common lead microstructure
IV.
CONCLUSIONS
This study developed a fabrication procedure to fabricate the mechanical locking micro-anchor structure to enhance the bonding between EMC and lead frame copper substrate for aQFN application. Through this method for developing anchor-shaped lead, it was capability of designing the more smaller line space and pitch. According to the simulation result of load-displacement plot, this micro-anchor structure developed can improve 2203
the bonding between lead and EMC. The pull-off force which the common lead microstructure and the microanchor structure were pulled off from EMC were around 2.09 N and 3.53 N, respectively. By the effect of mechanical locking of this micro-anchor structure, the pull-off force was raised by 69%. The interface adhesion strength would not change. Therefore, the mechanical locking effect could increase the rigidity of the aQFN package to reduce the relative motion of the interface under loading. It has a highly potential as the solution for preventing the delamination occurs.
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ACKNOWLEDGMENT This work was supported by the Advanced Semiconductor Engineering (ASE) [grant number: N104171] and the Ministry of Science and Technology (MOST, Taiwan) [grant number 104-2218-E-110-009-]
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