Method for applying double-sided reinforcing (DSR) material to a device

Abstract

Method for applying double-sided reinforcing (DSR) material to a fixture during an assembly process, the method comprising: Printing solder paste material onto a substrate of a printed circuit board; Mounting a ball grid array device with a pick-and-place machine; Immersion of the device in a pressure flux package-on-package (PoP) machine; Arrange the device on a solder paste pad on the substrate; Applying heat to the substrate to remelt the device in order to attach the device to the substrate; and Filling a gap between the device and the substrate with curable DSR material; wherein the DSR material comprises epoxy and curing agent; and wherein the combination of epoxy and curing agent triggers a crosslinking reaction only at elevated temperatures once the solder melts.

Description

AREA OF TECHNOLOGY

[0001] One or more aspects generally concern the electronics industry and, in particular, materials and processes for semiconductor packaging. BACKGROUND

[0002] There is a growing demand for innovative products that can be used as alternatives to conventional underfill materials in semiconductor packaging, both at the board and component levels. Switchgear manufacturers are seeking alternative, highly stable underfill materials that facilitate the easy processing of packaging materials in the electronics industry under various conditions.

[0003] JP 2003-059970A describes a packaging structure and packaging method for electronic components.

[0004] DE 101 45 826 C1 describes a method for connecting a component to a substrate and suitable storage containers for carrying out this method.

[0005] US 2004 / 0234689A1 describes a method for using prefabricated underfill encapsulations. SUMMARY

[0006] Double-sided reinforcement materials and methods are described according to one or more aspects.

[0007] The present invention relates to a method for applying a double-sided reinforcing (DSR) material to a fixture during an assembly process. The method according to the invention comprises printing a solder paste material onto a substrate of a printed circuit board, picking up a ball grid array fixture with a placement device, immersing the fixture in a pressure flux package-on-package (PoP) machine, arranging the fixture on a solder paste pad on the substrate, applying heat to the substrate to remelt the fixture in order to attach the fixture to the substrate, and filling a gap between the fixture and the substrate with curable DSR material, wherein the DSR material comprises epoxy and a curing agent, and wherein the combination of epoxy and curing agent triggers a crosslinking reaction only at elevated temperatures once the solder melts.

[0008] The method can further include, after applying heat to the substrate to remelt the device, completely filling any gap between the device and the substrate with curable DSR material. The method can further include analyzing the curable DSR material under the device using a scanning electron microscope.

[0009] Furthermore, a double-sided reinforcing material is described that exhibits the properties of backer rods with improved stability at room temperature. In one embodiment, the material comprises a mixture of resin, hardener, catalyst, and at least one other additive, all of which are stable at room temperature.

[0010] Examples of double-sided reinforcement material also include material that achieves the mechanical properties of conventional backer materials and / or material that achieves the drop / shock properties of conventional backer materials.

[0011] Further embodiments and advantages of these exemplary embodiments are discussed in more detail below. Furthermore, it is clear that both the preceding information and the following detailed description are merely illustrative examples of various embodiments and are intended to provide an overview or framework for understanding the nature and characteristics of the claimed embodiments. The accompanying drawings are included to provide a representation and further explanation of the various embodiments and are incorporated into this patent specification, forming part thereof. The drawings, together with the rest of the patent specification, serve to explain the principles and processes of the described and claimed aspects and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In the drawings, the same reference numerals generally refer to the same parts in all different views. Likewise, the drawings are not necessarily to scale; rather, the emphasis is on illustrating the principles of the disclosed embodiments and are not intended as a definition of the limits of such embodiments. For clarity, not every component may be labeled in every drawing. The following description details various embodiments with reference to the following drawings, of which: Fig. 1 a flowchart of a method for applying a double-sided reinforcing (DSR) material according to one or more embodiments; Fig. 2 is a photograph taken with a scanning electron microscope (SEM) of a package filled with DSR material according to one or more embodiments; Fig. Three case / shock data sets are shown, which are discussed in an accompanying example; Fig. 4 is a graph showing the viscosity stability of a DSR material according to one or more embodiments; Fig. 5 and Fig. 6 data from a temperature cycling test are shown, which are discussed in an accompanying example; and Fig. Seven case / shock data are shown, which are discussed in an accompanying example. DETAILED DESCRIPTION

[0013] The various embodiments described here are not limited in their application to the design details and component arrangements set forth in the following description or shown in the drawings. One or more embodiments can be implemented or carried out in different ways beyond those illustrated here as examples.

[0014] Currently, conventional thermosetting polymer-based underfill materials are commonly used as packaging materials to improve or enhance the mechanical properties of the material. Underfill materials commonly used in the encapsulation process in the electronics industry are temperature-sensitive and can cause problems when exposed to room temperature, such as premature hardening. This, combined with exposure to air, can lead to viscosity buildup in the underfill material, resulting in processing problems. Therefore, the use of conventional underfill materials has several disadvantages, as a separate curing process is required in addition to a standard remelting process, and the underfill materials must be stored below -4 °C.

[0015] According to one or more embodiments, materials and methods can overcome these disadvantages of conventional backer materials. In some embodiments, the disclosed material can be applied and serves as a conventional backer material and can be used in standard remelting processes to harden the material. After hardening, the material tends to have the properties of a backer material, exhibits room-temperature stability, and can be stored and used without additional measures. The disclosed DSR material is a material that has excellent room-temperature stability. In some non-limiting embodiments, the DSR material is stable at room temperature for at least twenty days. The disclosed materials are also stable in terms of viscosity. An advantage is that no separate hardening process is required apart from standard remelting processes.

[0016] As used here, remelting can generally be defined as a process in which solder paste is printed or dispensed onto a surface of a printed circuit board, or a solder preform is placed on it, or both of these occur, components are placed in or near the deposited solder, and the assembly is heated to a temperature sufficient to remelt the solder.

[0017] According to one or more embodiments, a double-sided reinforcing (DSR) material can exhibit properties of conventional backer rods but with improved stability at room temperature. In at least some embodiments, the DSR material contains a mixture of a resin, a hardener, a catalyst, and at least one other additive, all of which are stable at room temperature.

[0018] According to one or more embodiments, methods for applying the DSR material are disclosed. One method for applying the DSR material includes printing solder paste material onto a substrate of a printed circuit board, picking up a ball grid array device with a placement apparatus, immersing the device in a pressure flux package-on-package (PoP) machine, and arranging the device on a solder paste pad on the substrate.

[0019] The term “double-sided reinforcement material” or “DSR material,” as used here, can refer to a type of flux-containing curable composition. The composition may contain one or more resins, a hardener, a catalyst, and any other necessary additives. A fluxing agent may or may not be present. In at least some embodiments, no fluxing agent is present. When the DSR material is exposed to heat, such as through a remelting process, the material can undergo a three-dimensional crosslinking reaction to form a hard solid mass and the solder joint and any gap between the BGA spheres and the substrate, thus acting as an underfill material.

[0020] According to one or more non-restrictive embodiments, a DSR material may contain one or more of the following component(s) at the specified weight percentages: a) 30 - 40 wt% organic solvents with high boiling points b) 5 - 10 wt% epoxy resins of varying functionality c) 15 - 30 wt% solid high molecular weight bifunctional epoxy resins d) 3 - 10 wt% dicarboxylic acids as activator e) 2 - 8 wt% substituted aromatic amine as catalyst f) 1 - 5 wt% phosphene-based salt as catalyst g) 1 - 5 wt.% liquid, anhydride-like hardener / catalyst h) 0.1 to 4 wt.% liquid strain modifier i) 0.1 - 3 wt.% adhesion promoters j) 20 - 50 wt.% reinforcing fillers

[0021] According to one or more embodiments, the stability and desired viscosity of the material can be attributed to the ratio between the type of epoxy resin and the curing agent. The catalyst used should generally be weakly reactive at room temperature, and crosslinking should only occur at elevated temperature (processing temperature). Preferably, crosslinking should begin as soon as the solder melts. If the epoxy undergoes crosslinking before the solder melts, the crosslinked epoxy can harden, and the hardened epoxy may prevent the molten solder from forming a proper bond on the electronic circuit board. According to the invention, the combination of epoxy and curing agent triggers the crosslinking reaction at elevated temperatures as soon as the solder melts.The hardened / crosslinked epoxy can form a ring around the solder joint and can reduce the transmission of a load applied to the solder joint.

[0022] In at least some embodiments, the components can be mixed in a desired ratio and subjected to milling, such as three-roll milling. After confirmation of, for example, FOG <10 microns, the milling process is stopped. The milled sample can then be mixed with a desired amount of reinforcing fillers to obtain a finished DSR product.

[0023] According to one or more non-restrictive embodiments, the viscosity of the DSR material can be in the range of approximately 100 to 500 Pa·s. The calorimetric properties of the DSR materials can be measured by DSC to determine the peak curing temperature, which should generally be above the solder melting point. The glass transition temperature (Tg) and the coefficient of thermal expansion (CTE) of the DSR material can be measured using a thermomechanical analysis (TMA) instrument, and the storage modulus of the material can be measured by dynamic mechanical analysis (DMA).

[0024] According to some embodiments, a method is provided for applying a DSR material that can be easily applied by a PoP machine. The DSR material, intended to serve as an underfill material, may have non-flowing properties. After application of the DSR material (sometimes referred to as a "flux") by PoP, the DSR material can undergo crosslinking under a standard SMT remelt.

[0025] Fig. Figure 1 shows an exemplary embodiment of a method for applying DSR material. As shown, the method can involve printing a solder paste material onto a substrate of a printed circuit board, picking up a ball grid array device with a placement machine, immersing the device in a pressure flux package-on-package (PoP) machine, and arranging the device on a solder paste pad on the substrate. After the remelting process, a gap between the component and the board can be completely filled with curable DSR material.

[0026] According to one or more embodiments, various process parameters can be controlled. These can include, for example, immersion time, immersion depth, and the rise rate of the immersed component. The immersion time can be significant in that it determines the amount of flux transferred to each solder ball in the BGA. The amount of flux transferred to each ball can influence the final mechanical reliability characteristics of the assembled package. A longer immersion time can lead to higher mechanical properties of the package. Typically, the immersion time can vary from smaller to larger packages. In some non-restrictive embodiments, an immersion time of approximately 0.1 seconds to approximately 5 seconds may be recommended. The immersion depth can also contribute to achieving high mechanical strength in the finished package.In some non-restrictive embodiments, the immersion depth can vary from approximately 50% to approximately 90%, depending on the package and the final mechanical reliability requirement for that package. In some specific embodiments, approximately 90% of the BGA package ball height is recommended as the immersion depth. The ascent rate is generally known as the rate at which the device lifts the component from the flux-filled DSR material tray. The ascent rate should be optimized for different package types prior to machining.

[0027] According to one or more embodiments, a DSR material has the properties of conventional backer rods, but with excellent stability at room temperature. Conventional backer rods are not stable at room temperature, and their viscosity is impaired, which negatively affects their flowability and curability during processing. The DSR materials disclosed today are stable at room temperature and tend to exhibit all the favorable mechanical properties of backer rods.

[0028] The process and compositions described herein can be used in applications including, but not limited to, printed circuit board manufacturing, LED assembly, photovoltaic cell manufacturing, semiconductor manufacturing and die mounting.

[0029] The function and advantages of these and other embodiments of the materials and processes disclosed herein will become clearer from the following examples. The following examples are intended to demonstrate the advantages of the disclosed materials and processes, but do not illustrate their full scope. EXAMPLE 1

[0030] The analysis of a curable DSR material under one component was performed using a scanning electron microscope (SEM), as described in Fig. Figure 2 shows that the gap between the plate and the BGA component is completely filled with curable DSR material and that no pores are present. This pore-free, completely filled DSR material is responsible for improving the mechanical reliability of the components on the plate. EXAMPLE 2

[0031] The drop / shock properties of the DSR material were compared to a standard underfill material, Stay Chip 3082, as described in Fig. 3 shown.

[0032] The Weibull curve of Fig. Figure 3 shows drop / shock data for a standard SMT solder paste, a standard SMT solder paste with Staychip 3082 underfill material, and a standard SMT solder paste with DSR material (NHHV4+40% Si). The comparison clearly shows that the drop / shock resistance of a solder paste with underfill material is much higher compared to the standard solder paste.

[0033] Furthermore, it is also confirmed that the DSR material is equivalent to the underfilled and remelted plate. This drop / shock data provides strong confirmation that the DSR material behaves like an underfill material. EXAMPLE 3

[0034] The stability of the DSR material was determined using a Malcolm viscometer, and the viscosity of the DSR material was measured at regular time intervals. The viscosity data from the Malcolm viscometer are presented in Fig. Figure 4 shows that the DSR material exhibited excellent stability. EXAMPLE 4

[0035] A temperature cycling test was performed according to the IPC 9701-A standard (0°C (10 min) to +125°C (10 min) over 1000 cycles). Cross-sectional and microscopic observation of BGAs after the 1000 cycles were carried out for fault analysis. A fault was defined as a 20% increase in nominal resistance within a maximum of 5 consecutive read scans. The data are in the accompanying document. Fig. 5. As shown, the electrical resistance did not increase by more than 20% for any of the pastes. Fig. Figure 6 shows the cross-sections after temperature cycling.

[0036] Both SH2 and NHHV4-S withstood 1000 temperature cycling cycles from 0°C to +125°C (10-minute dwell time). No cracks were observed on the solder ball or flux material of SH2 and NHHV4-S after up to 1000 cycles. In contrast, cracks appeared after 750 cycles of the Sn3Ag0.5Cu CVP390 solder paste without the addition of SH2 or NHHV4-S. Thus, SH2 and NHHV4-S provided the solder joint with additional resistance to cyclic expansion (caused by a CTE mismatch) during the temperature cycling test. EXAMPLE 5

[0037] The drop / shock properties of the disclosed DSR materials were compared with those of a conventional solder paste according to the JEDEC standard JESD22-B 111 "Board Level Drop Test Method of Components for Handheld Electronic Products" with service condition B (1500 Gs, 0.5 ms pulse, half sine wave). A fault detection was defined as the first event of an intermittent discontinuity, followed by 3 additional such events during 5 subsequent drop events.

[0038] The data is in Fig. 7. The NHHV4-S and SH2 drop / shock characteristics were 2x and 3x higher, respectively, than with conventional solder paste alone. The SH5-F drop / shock characteristic was almost 6x higher than with conventional solder paste alone.

Claims

[1] Method for applying double-sided reinforcing (DSR) material to a fixture during an assembly process, the method comprising: Printing solder paste material onto a substrate of a printed circuit board; Mounting a ball grid array device with a pick-and-place machine; Immersion of the device in a pressure flux package-on-package (PoP) machine; Arrange the device on a solder paste pad on the substrate; Applying heat to the substrate to remelt the device in order to attach the device to the substrate; and Filling a gap between the device and the substrate with curable DSR material; wherein the DSR material comprises epoxy and curing agent; and wherein the combination of epoxy and curing agent triggers a crosslinking reaction only at elevated temperatures once the solder melts. [2] Method according to claim 1, wherein the filling of a gap between the device and the substrate with curable DSR material is carried out after applying heat to the substrate to remelt the device. [3] Method according to claim 3, further comprising analyzing the curable DSR material under the device using a scanning electron microscope. [4] Method according to claim 1, characterized by an immersion time of approximately 0.1 seconds to approximately 5 seconds. [5] Method according to claim 1, characterized by an immersion depth of approximately 50% to approximately 90%.

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