Solutions for controlling resin bleed out
The hows and whys of resin bleed-out (RBO) are discussed, as well as the impact it makes and how to control it.
BY RONGWEI ZHANG, ABRAM CASTRO and YONG LIN, Semiconductor Packaging, Texas Instruments Inc., Dallas, TX
Die attach pastes, which consist of resin, curing agent, catalyst, filler and additives, have been extensively used to attach die onto lead frames in various electronic packages such as small outline integrated circuit (SOIC), thin-shrink small outline package (TSSOP), quad flat package (QFP) and quad-flat no-lead (QFN). One of the issues commonly encountered during package assembly is resin bleed-out (RBO), or epoxy bleed out (EBO). RBO is the separation of some formulation ingredients in the paste from the bulk paste (see FIGURE 1). Depending on die attach paste formulations and lead frame surface chemistry and morphology, bleeding ingredients can be solvents, reactive diluents, low-molecular-weight resins, catalysts, and additives like adhesion promoter. Resin bleed out tends to occur on high energy surfaces such as metal lead frames without any organic coating. In particular, if plasma cleaning is utilized to remove the contaminants prior to assembly, the bleeding issue may become more pronounced due to the increase in surface energy. Bleed-out can occur once die attach pastes are dispensed on to lead frames or during thermal curing. As microelectronics continue to move towards smaller form factor, higher reliability and higher performance, control of RBO becomes increasingly critical for packages where there is a very little clearance between die and die pad edge, or between one die and another in multi-chip modules (MCMs).
How resin bleed-out occurs
When die attach paste is dispensed onto a solid surface like lead frame surface, the paste will typically wet the surface partially. The adhesive force between die attach paste and lead frame surface causes the paste to spread while the cohesive force within the bulk paste will hold the ingredients together and avoid contact with a lead frame surface. The adhesive and cohesive forces are the intermolecular forces such as hydrogen bonding and Van der Waals forces. So the degree of wetting will depend on the balance between adhesive force and cohesive force. Bleed-out occurs when the adhesive force of some formulation ingredients to the substrate is stronger than the cohesive force within the paste. The driving force for bleed out is to minimize the surface energy of the substrate by wetting.
Impact of resin bleed-out
Resin bleed-out can cause several issues if it is not well controlled.
• If the formulation ingredients bleed from the periphery of the die attach pastes and covers the wire bonding area, then issues like non-stick on pad (NSOP) and weak wire bond can occur. It can also be an issue if bleeding occurs from the die attach fillet along die edge to the die top, contaminating the bond pad on die top surface .
• Resin bleed-out may affect the adhesion of mold compound to die pad or mold compound to die top surface, both of which can lead to delamination. In particular, die top delamination is strictly not allowed in wire-bonded packages because it can cause the ball bond to be mechanically lifted, thereby leading to electrical failures during temperature cycling .
• As the formulation ingredients bleed out of the bulk paste, the composition of die attach paste under die may change accordingly. This can impact the adhesion of die attach to lead frame adversely, leading to an adhesive failure .
Influence of surface roughness
There are many factors that can cause resin bleed-out, such as low surface tension of die attach pastes, high surface energy of metal lead frames, surface contami- nation, surface porosity and surface roughness. Here we will focus on the impact of surface roughness, which is critical to achieve high package reliability. Two die attach pastes were dispensed onto three lead frames with different surface roughness. The surface roughness of these three lead frames was characterized by Atomic Force Microscopy (AFM) using the roughness average (Ra) and the roughness ratio (r) (FIGURE 2). The roughness average (Ra) represents the arithmetic average of the deviations from the center plane. The roughness ratio is the ratio between the actual 3-D surface area calculated by AFM and the flat surface. The 3D morphologies of lead frames are shown in FIGURE 3. It was found that (a) there is a good correlation between the roughness ratio and resin bleed-out. As the surface roughness ratio increases, the bleeding becomes increasingly worse; (b) LF1 and LF2 have almost same Ra, but the bleeding performance of DA3 and DA4 are different. This indicates that the roughness average is not a good index for RBO; (c) DA4 is more resistant to bleed out than DA3.
The relationship between surface roughness and the wettability has been described by Young equation (Equ. 1) and Wenzel equation (Equ. 2).
Where Ys, YL, YSL are surface tensions of the solid, liquid and interfacial tension between die attach paste and lead frame, respectively; 0y is the Young contact angle, 0m is the measured contact angle, and r is the roughness ratio. As the surface roughness increases, the better the wetting, and the worse the bleed-out if the contact angle is < 90o . This is the case for die attach paste on a metal surface without anti-EBO coating.
Approaches to control resin bleed out
There are several approaches to control or eliminate resin bleed-out. These approaches include modifying formulation by selecting appropriate anti-EBO, using die attach film (DAF)/B-stage epoxy, controlling surface roughness, creating mechanical barrier, and lowering the surface energy of lead frames by surface coating.
• Modifying formulations. Generally, anti-bleeding agents are added to die attach pastes to reduce or eliminate RBO. Different anti-bleeding agents may have different working mechanisms. Some anti- bleeding agents are added to enhance the cohesiveness of the pastes while others are added to form a thin layer with a surface energy lower than the pastes themselves on a lead frame surface . Therefore, tailoring die attach adhesives with appropriate anti-bleeding agents is critical to prevent RBO on different types of lead frames, while maintaining high adhesion to metal lead frames to achieve high reliability.
• Die Attach Film/B-stage Epoxy. The simplest and most effective way to eliminate RBO is to use die attach films or B-stage materials. However, there are limitations associated with this approach. These can include high material cost and capital investment, difficulty to achieve high adhesion and thus high reliability, and limited thermal performance of these materials.
• Mechanical barriers. In some cases, grooves on lead frames are designed in between die attach area and wire bond area to reduce resin bleed-out, as shown in FIGURE 4. This is a simple and cost-effective process. However, this approach may not work well if the bleeding is severe. Similarly, some low surface energy insulating film around a chip can be printed to confine the un-cure pastes to the space defined by the printed pattern .
• Vacuum baking. Vacuum baking of ceramic substrates with gold or other metal surfaces has been reported to reduce bleed-out. Several mechanisms were proposed: (a) through removal of polar surface contaminant, which promotes bleed-out of lighter organic resin by dipole attraction or chemical reaction ; (b) through reducing the surface energy of the plating surface by the formation of Ni2O3 ; (c) through producing a coating of hydrocarbon by oil back streaming toreduce the surface energy . The method is not recommended either due to lack of controllability or due to the detrimental effect on wire bonding quality . A more controlled method to reduce or eliminate RBO is to treat the surface with known chemicals and controlled processes, as discussed below.
• Low surface energy coating. Roughened lead frames have been utilized to enhance package reliability, particularly to meet Automotive Grade 0 requirements or beyond, as they increase surface contact area and enhance mechanical interlocking. As shown in Fig. 2, a small increase in roughness can result in a severe bleed-out. Therefore, increasing surface roughness will promote bleed-out if there is no anti-EBO on the surface. According to Young’s equation, decreasing surface energy will increase the contact angle, i.e. decreasing the wetting of the surface. Therefore, in roughened lead frame manufacturing, a solution of low surface energy material is used to treat roughened lead frames to lower their surface energy to reduce or eliminate RBO. Alternatively, a thin layer of film can be deposited onto the assembly surface by gas plasma technology to modify the surface energy . FIGURE 5 shows water contact angles of lead frames with or without anti-EBO treatment. The anti-EBO coating will increase the contact angle on standard lead frame as explained by Young’s equation. Compared with standard lead frames, roughened lead frames have an increasing roughness and the anti-EBO coating on roughened lead frames further increases contact angle significantly. This can be explained by Wenzel equation, which demonstrates that adding surface roughness will increase surface hydrophobicity if the surface is chemically hydrophobic. In addition, Fig. 5 shows the resin bleed-out performances of a die attach paste (DA2) on these three types of lead frames. Bleed out was observed on the standard lead frame without anti-EBO, but there was no bleeding on both standard and roughened lead frame with anti-EBO coating. The low surface energy anti-EBO coating eliminates resin bleed out.
This article provides an understanding of how bleeding occurs, the impact of bleeding, and methods to control bleeding. Bleeding is the result of the interaction between die attach pastes and metal lead frames. In particular, we studied the influence of surface roughness on RBO of different die attach materials, and found that there is a good correlation between the roughness ratio and bleed-out performance. Reducing the surface roughness will reduce or eliminate RBO. It is noteworthy that there is a line between reducing roughness to achieve no RBO and increasing roughness to ensure excellent delamination performance for lead frames without Anti-EBO. In terms of die attach pastes, the most effective way to control RBO seems to be the surface coating with anti-RBO without affecting other performances like delamination, or combining this method with others to provide an even better solution.
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