Chip Scale Review - November December 2019

10 Chip Scale Review November • December • 2019 [] Fluxless wafer bumping by micro-ball placement By C. Christine Dong [Air Products and Chemicals, Inc.] afer bumping is a process of forming solder bumps on electrode pads of entire wafers. This process is often divided into two categories: one is for flip-chip (FC) packaging, and another is for wafer-level chip-scale packaging (WLCSP). Bumps on flip-chip packages are typically in the range of 50 to 200µm in height, while bumps for the WLCSPs are normally in the range of 200 to 500µm in height. Commonly used solder deposition methods for wafer bumping are paste printing, electroplating, and ball placement. For each different application, a suitable solder deposition method must be selected. For example, paste printing has a low cost, but cannot be applied for fine- pitch micro bumps. In addition, voiding is inevitable due to a significant amount of nonmetallic content inside the paste. Electroplating is more suitable for depositing fine-pitch micro bumps, however, it is relatively expensive, and it is not suitable when bump size is larger than a certain value. As a comparison, the ball placement method for solder deposition can be applied for a much larger range of bump sizes, such as from 50µm to 500µm. Other key advantages of the ball placement method include high throughput, good bump uniformity, low cost, and reduced chemical pollution. The first step of the ball placement process is depositing an organic flux on the wafer pad areas, which can be done by pin transfer or stencil printing. Preformed solder balls are then placed on the flux deposits all over the wafer simultaneously, such as by stencil printing or vacuum transfer. After the ball placement, the whole wafer goes through a reflow step. During this step, the initial oxides on the surfaces of the solder balls and electrode pads are cleaned by the organic fluxes, and then the solder balls are melted, thereby forming a metallic bond between the solder bumps and metal pads. The reflowed wafers will also need to go through a post- cleaning step to clean flux residues that remain on the wafer surface. With the industrial trend moving toward device miniaturization, the pitch size of the solder bumps is continually shrinking. As a result, flux residues tend to be trapped between solder bumps, making the post- cleaning step ineffective. Therefore, a fluxless technology is highly desirable. The current study is related to a new technology of wafer bumping that is achieved by micro- ball placement without using organic fluxes. More specifically, a residue-free locating agent is developed to hold the solder balls placed on the electrode pads. In addition, a production-scale reflow furnace containing a hydrogen activation function is used to reflow the wafers in a non-flammable gas mixture of hydrogen (<4 vol%) in nitrogen. Initial oxides on solder b a l l s a nd e l e c t r o d e pads can be effectively r e mo v e d u n d e r t h e activated hydrogen at ambient pressure and below the solder’s melting point. It is demonstrated in this study that by using this fluxless approach, a good solder ball wetting can be achieved, and the surface of the reflowed wafer is free of foreign materials without the need of the post-cleaning step. Experimental method The wafers used i n this st udy to demonstrate the fluxless wafer bumping p r o c e s s by m i c r o - b a l l p l a c eme n t were supplied by a customer in the semiconductor industry. Metal layers were uniformly formed on the entire silicon wafer surface with a plated copper layer on the top ( Figure 1 ). The thickness and composition of each metal layer were made exactly the same as that of electrode pads on a real product wafer that this customer would typically use. Each wafer was diced into 25mm X 25mm squares as substrates for ball placement and reflow tests. Atomic force microscopy (AFM) was used to check the roughness of the plated copper surface on the wafer ( Figure 2 ). The root-mean- square roughness value for the copper surface is around 20nm. For proving the concept, 400μm solder balls with a composition of SAC305 (3.0% silver and 0.5% copper to tin balance) were used in this study. The melting point of the solder is 217 to 220°C. A proprietary locating agent was developed to replace the organic fluxes for holding each solder ball on the copper substrate. A pin transfer method was used to deposit the locating agent. More specifically, a fixture containing an array of 16 X 16 metal pins was made to transfer the liquid from a container to each copper substrate. After depositing the locating agent, solder balls were placed on the liquid deposits by using a metal sieve. A production-scale ref low furnace containing the hydrogen activation function was used in this study. More specifically, the activation of hydrogen gas molecules W Figure 1: Non-patterned copper wafer used in the ball placement study herein. Figure 2: A surface roughness on the copper wafer of about 20nm.