Fine Pitch Paste for System-in-Package Applications Open Access

Solder electrical interconnect is a metallurgical bond system that consists of chip metal pad, under bump metal and solder ball. It is important to understand the solder interconnection structure and characteristics for a long-term reliability and shorter-term assembly considerations. During the last two decades, integrated-circuit technology has evolved dramatically from QFN to Chip-Scale-Packaging (CSP), System-in-Package (SiP), Package-on-Package (PoP), Wafer-Level-Packaging (WLP), 3D Through-Silicon-Via (TSV) technology, etc. For that, different kinds of applications are required to complete the packaging assembly process. One of the most widely used assembly materials is solder paste, a mixture of alloy particulates and organic resin flux applied to form electrical interconnects between component and substrate. Screen-printing is less expensive than evaporative wafer bumping and plating processes. Stencil printing of solder paste is promising for system manufacturers as it constitutes one of their most important steps in surface mount technology in board level application by millimeter or sub-millimeter level. Where, alloy particulates in micron size level are developed to adopt in SiP. It is a cost effective interconnects over electro-plating, versatile and reveal good printing performance to assemble wide range of component size (01005/0402 to 008004 imperial code) with increasing component density on a single board. Recent developments in flip chip technology printed 60~80μm copper pillars with solder tips. Such pad sizes ranges from 80 to 100μm. Generally, the type of SAC (Sn-3wt%Ag-0.5wt%Cu) powder used in solder paste are defined as Type4 (25–38μm), Type5 (15–25μm) Type6 (5–15μm), Type7 (2–11μm) and Type8 (2–8μm), as per IPC J-STD-005 standard. Industry is yet to move towards finer sub-micron size. Powder characteristics such as surface area, total oxide, carbon content, spheroid nature, size distribution etc., influence the paste behaviors. The scope of the present study is to investigate the influence of SAC305 powder distribution and volume fraction addition on the rheology of T7 solder paste, and microstructural observation of the bond interface.

Welco processing of solder powders is an established technology in our production plant [1, 2]. Basically, powders are processed shearing the molten SAC305 dispersed in a liquid medium having melting point slightly higher than solder of about 220°C. Rate of shearing, design of rotor-stator, vortex of SAC305 dispersed medium, bath temperature, rotor speed, shearing cycle (time), and quantity of molten metal controls the distribution of powder particles. Fig. 1 shows typical distribution of powder particles of T5, T6, T7 and T8 processed using this method. The distribution is well within the recommendation of IPC standard for T5 and T6. Welco process is the best to produce spherical, un-agglomerated particles with clean and smooth surface as shown in Fig. 1. The oxygen content of T7 powder particles is measured using instrumental gas analysis (IGA) and found it to be in the order of few hundred ppm. Soldering is demonstrated to be good for the reported oxygen level.

The factors that influence good screen printing are stencil type, stencil thickness, squeegee angle, printing pressure, print speed, print gap, separation speed, aperture opening type and size, etc. Stencil processed by electroform using nickel alloy of 30 to 50μm thickness is currently practiced. Stainless steel squeegee printing at an angle of about 60°, maintaining zero printing gap between stencil bottom and board top (or slightly negative) provided good printing. Both square and circular aperture opening are examined. The separation speed of squeegee, printing speed and pressure are opted depending on the printer model and the type of paste used. Presently, WS paste processed with T7 powder of SAC305 is printed using DEK Horizon 03iX at 25mm/s print speed and 1mm/s squeegee separation speed with 3mm separation distance.

Solder paste was cleaned using deionized or distilled water maintaining halogen free activity level. The paste was thawed for 2h at room temperature before printing, where the temperature ranges from 23 to 27°C and relative humidity (RH) from 50 to 60%. Viscosity was measured using 25mm spindle of Anthon-Paar rheometer for 10 shear rate viscosity and 0.3 shear rate yield point at room temperature. Tackiness was tested using Malcom tester. Bridging was observed using Keyence VR-3200 wide area 3D system. The paper enumerates the experimental research on the novel T7 solder paste used for fine pitch application, denoted as WS-T7.

When WS-T7 solder paste is printed on copper substrates for 8h, data for every 2h is collected on paste rolling behavior, paste viscosity, yield point, tackiness, number of bridging when printed with 50μm line spacing, etc. Yield point is point of deflection from constant slope (elastic) to exponential slope of a plot of shear stress versus shear rate (Fig. 2). Table 1 shows the rheological behavior of paste as stencil life for 0 to 8h, samples are tested every 2h of the printing cycles. Shear thinning is evident in the initial period of printing say 2h. The paste rolling is smooth and excess hang-up of paste on the squeegee is absent for the tested printing cycles until 8h (Fig. 3).

Printing WS-T7 on copper surface for 50μm line spacing and larger pad opening between 100 and 120μm revealed bridging. However, bridging is absent for 70μm pad opening (Fig. 4, Table 2). Square aperture demonstrated relatively more bridging than circle when printed from 0 to 8h cycle time (Table 2). Stable paste release is observed (Fig. 5) for 70μm pad opening and printing from 0 to 8h. Similar results are observed for three batches of printing tests.

WS-T7 solder paste is printed on copper-OSP fine pitch test board 100 × 50 × 1.6 mm (Fig. 6). The copper surface finish is 1oZ thickness, its surface showed the following characteristics:

• Provided solderable surface for joints,

• Achieved self-alignment of solder joint during the reflow,

• Have controlled collapse of solder during the reflow process,

• Protected the chip from damage or degradation

Measuring the angle of contact of solder to copper surface on either sides showed acute angle, exhibiting excellent wetting and spreading of molten WS-T7 solder. Microstructural observation of the interface at high magnification of 2000X using scanning electron microscope (SEM) revealed molten metal reaction between solders and copper surface by forming scallop of CuSn intermetallics. Line scan energy dispersive X-ray (EDX) analysis confirmed the scallop is CuSn intermetallics, identifying the presence of both Cu and Sn elements for the entire length of intermetallic scallop of 3μm height. WS-T7 paste is also printed on plated surfaces of gold (ENIG) and tin (immersion) as well reflow showed good solder balling, wetting without splashing. As expected gold and tin surface shows good wetting similar to copper surface (Figs. 7 & 8). The reflow peak temperature is set to 248°C, purging nitrogen and maintaining 150 ppm oxygen, the convection oven has eight heating zones to retain the alloy in a molten state (liquidus time) for 72s. Table 3 provides the measured angle of contact of solder with different plated surface (Cu, Au, Sn) and pad/substrate shapes (Figs. 6 to 10).

Soldering fine components of 01005 and 008004 with WS-T7 paste to gold plated surface revealed zero voids, absence of splashing/bridging/beading/tilting. Consistent paste release is evident in printed and reflow soldered 2800 components. X-ray imaging of the solders after two consecutive reflows and purging nitrogen revealed zero voids in all the inspected 50 components (Fig. 9). Soldered components at zero and 8h staging both showed dense void free solder attachments without any tilt or movement of the components. Intermetallics containing tin and nickel are observed on reflow. Cross sectioning the solder bump processed with solder mask revealed integral bond interface (Fig. 10).

Solder paste is a homogeneous mixture of metal alloy powder (90wt%) and flux (10wt%) which is made of organic chemicals. The ratio of alloy powder and paste flux could be slightly different in terms of powder size, alloy composition and paste formulation and application. For dispensing, fluidity of the paste is different from printing with high amount of flux in the paste. It can be from 10% to 20% flux in the paste to increase the fluidity. However, the function of flux remains the same to possess cream-like texture and enables the formation of metal bump joints without oxidation during reflow. To promote wetting and to form strong interconnect flux functions are to:

1. Remove the solder ball surface oxide layer creating active and wettable solder alloy

2. Prevent the clean surface from re-oxidation before wetting

3. Remove the reaction product from the surface and allow the solder alloy to contact the base metal surfaces (substrate and pad)

Although, first is the principal function yet the other two are important for the paste to perform. Hence, needs a good understanding on flux formulation and its chemical activity. Rosin resin plays a vital role in flux, contributing to paste tackiness, viscosity, printing properties, removing the solder ball surface oxidation as activator during reflow, covering the cleaned surface to prevent further oxidation as well. In addition to rosin, other activators are also added in flux such as amines, acids, halide/halogen, etc. The main function of the activators in a flux is their efficiency to clean the oxide compound at high temperature. Same time, have no reaction or slow reaction with alloy powder at room temperature. Hence, the paste has a longer shelf and stencil life. Organic carboxylic acid (R3COOH) in activator can clean copper oxidation and solder surface oxidation. Longer chain organic acid has more stable shelf life than paste with short chain acid. The amount and type of organic acid added in formula affects the paste stability significantly. Several trials helps to find a compatible activator.

The main function of these rheological additives is to provide a paste to suit syringe dispensing and stencil printing, helps to prevent separation between flux and powder particulates in paste. There are two key types of rheological additives, hydrogenated castor oil and polyamide types, where the activation temperature and time are different, some need external force to activate. Solvents in flux are basically alcohols, ethers, glycols, etc. They are used to dissolve activators, thickener, rosin and other additives in flux formula. As the largest volume fraction of flux, solvents not only serve as flux carrier, it also helps to protect the clean metal surface from re-oxidation. Moreover, it has significant effect on the tackiness and viscosity of paste, has impact on the stencil life and shelf life of the paste. In addition, to combination of flux and solder powder, paste and flux production process, production environment and storage can also have dramatic effects on the behavior of final solder paste product during printing and dispensing. Table 4 provides the powder particle size distribution, stencil life and paste tackiness for T4 - T7 powder size.

The developed solder paste behaves as all-in-one printing paste of passive components to terminal pads and flip-chip copper pillar pads printing with low voids. In flip-chip using printed solder pads reveal excellent inter-connection, good wetting without substrate warpage. Good solderability between solder tin and copper cap prevents solder creep in copper pillars. Both terminal and flip-chip pads are printed using a stencil in a step. This provides clean flip-chip pads without flux or dipping paste, simplifying the process of SiP assembly with reduced cycle time and cost.

Solder joint shape is another factor that could affect solder interconnect reliability. Fig. 11 shows typical solder bump structures as joint interconnections; mostly bump takes on the shape of a spherical structure. For the same solder material volume, chip and substrate pad sizes, hour-glass type solder joint has the highest standoff height. Deviations from the spherical shape can significantly increase into a stretched joint or decrease to a squashed joint affecting the lifetime of thermal cycle. Mathematical calculations and finite element modeling have shown that the shape of BGA bumps affects the stress-strain behavior in the BGA solder joints and a symmetric hour-glass-shaped solder bump experiences the lowest plastic strain and has the longest lifetime [3]. Generally, solder joint failure occurs first at the interfaces between solder bump and silicon chip and/or substrate. This is due to the high thermal stress concentration at these adhering interfaces, especially at the corners. It is commonly known that, for fatigue failure, fine crack starts at the point with high localized stress and gradually gets fractured where the stress is concentrated. Therefore, it is very important to reduce the stress to improve the reliability of solder joints. Among the several common solder joints shown in Fig. 11, the contact edges of the hourglass type shows the lowest stress at the joints with smaller contact angle. Thus, assembly lifetime can be improved by using hourglass shaped solder joints. The solder joints made by conventional solder bump methods are barrel shaped requires large pitch to obtain reliable joint heights with less stress at the interface.

The developed water soluble solder paste with T7 powder particles revealed:

• Good printability when printed on copper, tin, gold flash nickel plated surfaces

• Cross-section showed no obvious voids, good wetting and soldering to the plated surfaces

• Mostly on reflow the solder interconnect forms barrel type joint with an acute angle of contact in respect to plated surface

• All the bond solder interface are integral with substrate pads

• Tin based intermetallics formed at the interface on reflow

Authors acknowledge the production team for supporting the experiments and processing the solder paste. Also grateful to colleagues in materials characterization laboratory for cross sectioning the solder samples.