Reliability Design and Optimization Process on through Mold via using Ultrafast Laser

Abstract

In this study, a through mold via (TMV) process using ultrafast Laser has been developed for the interconnection on 3D package-on-package (PoP) application. Epoxy molding compound (EMC) is a composite material consisted of epoxy resin, filler particles (fused silica) and other constituents. Among these chemical compositions, the size and spatial distribution of fillers on the EMC dominate the sidewall quality on drilled trench. However, random filler leakage on the sidewall leads to an irregular and uneven trench and these drop out fillers become a problem needed to be solved. Faster removal of debris can be achieved by controlling the laser drilling parameters. Parametric studies have been performed to determine the specification of trench and design rule of material. The optimal desmear process on TMV bottom PoP package developed in this study demonstrates the reliability improvement on follow-up sputtering or filling process.

1. Introduction

Package-on-Package (PoP) using through mold via (TMV) technology was first introduced by Amkor at the 58^th ECTC conference in 2008 ¹ , and since then POP has become a preferred assembly process for higher IO density, smaller pitch, thinner overall size and lower warpage on 3D integration of logic and memory components in mobile products^1–5. In this mixed logic-memory stacking, based on FC BGA or PBGA style, the logic package is placed on the bottom and the chip-stacking memory package is located at the top ⁵ . The base of PoP structure contains ASIC and SMT passive components. After encapsulation process, a blind via was drilled through the Epoxy molding compound (EMC). This thin trench on bottom package would expose solder bump or metal bond pad on the substrate. The drilled vias were then sputtered/plated on the sidewall, or filled with the conductive material, The upper PoP structure including the stack-die memory with wire bond would be fully molded as a standard PBGA structure. These two packages then assembled as packed TMV PoP after final reflow process^1–2,5.

Mold compounds contain as many as 20 raw materials, such as organic resins (epoxy resin), non-melting inorganic filler (fused silica), catalysts, mold release material, pigment, flame retardants, adhesion promoters, ion traps and stress relievers … etc. Among these, filler occupied most of volume and made of non-crystalline SiO₂ which dominates mechanical behavior of compound. A smaller size of filler would result in a good spiral flow and a better reliability ⁶ . Over years, the modern mold compound has evolved into a complex formulation to meet the electronic industry needs. An innovation in PoP structure required a similar change in the design of the encapsulant, which filled with fused silica up to 90%. With affordable cost, the traditional flake silica was fabricated and changed to the modern spherical silica in which the size was dramatically reduced to keep pace with PoP structure trends. Size and spatial distribution of fillers in mold compound greatly impacted mechanical properties of EMC ⁷ .

Laser has the advantages of being able to machining irregular shape and finer edge quality, however, the laser beam influences on different materials to be process and debris becomes a problem to be solved. Since the formation of laser drilling is heat ablation, thermal effect is the basic physics. However, this also brings up a serious problem of heat affected zone (HAZ), which deteriorates the edge quality and material strength. As a via is drilled, EMC material ablated from the trench can be recast on the surface. Ejected fillers from the laser escaping through a deep, narrow trench would interfere with the incoming laser beam. In addition, loosened fillers on the irregular sidewall should block off the outer fringes of the laser, resulting in less laser beam reaching the bottom and less energy absorption. Consequently, the next step of drilling became much narrower width, which formed a constricting taper as the trench gets deeper. All of these effects served as the limitation to the depth of the trench created by laser machine.

Since the edge quality of sidewall on drilled via plays an important role for high speed interconnection, many researchers^8–12 have demonstrated many reliability design rules on the TMV bottom PoP packages in recent years. Among these, a so-called gas-aided laser technology ¹² was developed to improve the edge quality of sidewall on the trench. Oxygen (O₂) gas could increase the inflammation and noble gases, such as Argon (Ar) and Nitrogen (N₂) would decrease the oxidation during laser process. Compared with pure air, all gas-aided laser processes could improve the edge quality on the trench with additional production cost.

Random filler leakage on the sidewall was reported ¹² and the surface integrity of trench, such as debris, recast, ripple … etc, should be avoided during fabrication all the time. The ideal condition for trench formation is straight via with uniform width, however, the tapered via always formed due to the optical characteristic of aspect ratio. Figure 1 schematically illustrates the failure mode of non-uniformed width trench and drop out fillers are stuck on the narrow bottom is observed. A rough and uneven trench due to fall-off fillers would definitely deteriorate the follow-up sputter/plating process. Figure 2 demonstrates the SEM pictures of bumping uneven surface due to (a) remaining fillers (b) drop out fillers along the sidewall on the drilled trench. As can be seen, reliability issues occur because the remaining fillers or fall off fillers would either block sputtered/plated materials or lead to insufficient conductive fillings inside the trench.

Figure 1

Schematic illustration of failure mode on trench diameter with filler size

Figure 2

Bumping and uneven edge on sidewall due to (a) remaining fillers (b) drop-out fillers

TMV using ultrafast Green laser and IR laser has been developed in this paper. In the first, the definition of trench specification, especially the feature dimension of counterbore entrance for removing the bottom debris has been carefully determined. Secondary, the design rule of EMC material has been established for various micro-size spherical fillers. Fillers in EMC affect Laser process has been thoroughly examined. This study also explores the mechanism of faster removal of debris under various operating conditions. A series of comprehensive experiments of laser drilling on mold compound has been conducted in this paper.

2. Experimental Works

2.1 Laser Drilling Equipment

Two picosecond pulse duration lasers, Green laser (532 nm wavelength, 30W power, <1.3 m² beam quality, 150 μJ pulse energy and 200–800 kHz base frequency) and picosecond IR laser (1030 nm wavelength, 50W power, <1.3 m² beam quality, 250 μJ pulse energy and 200–800 kHz base frequency) were used to determine optimal parameters for vertical wall side on drilled via in mold compound. Compound thickness of 1000 mm, 200 mm and 100 mm with ultrafine filler size were tested to investigate the material behaviors after laser drilling. The cross-sectional profiles after each laser cutting were measured by 3D Laser Scanning Microscope (Keyence VK-X200) and FE-SEM (Hitachi-4700).

2.2 Trench Specification

Aspect ratio defined by laser beam has a great influence on the depth of cut. The aspect ratio of a laser drilled via is defined as the ratio of the hole's width to the hole's depth, i.e. final tapered depth/width at entrance. A narrow bottom width was normally formed for a large thickness of compound (1000 mm). Under highly controlled conditions, design rule for the width of bottom via and the minimum variation of entrance were determined.

2.3 Rule of Material

The shape of filler has been fabricated in round shape with diameter ranged from 25 mm to 50 mm for molding process. However, some of larger fillers were congregated in some area and spread out over the compound. Most of remaining fillers on side wall edge were found to be easily drop out into the bottom of the trench. Figure 3 illustrates the rule of material for laser beam entrance. Ideal straight, counterbore and countersink entrances have been studied for removal of fillers and follow-up interconnection.

Figure 3

Design rule of material for ideal straight trench and variation of entrance

2.4 Verification and Repeatability

The optimal parameters for desmear includes power, frequency (repetition rate), cutting speed, pass (cutting cycle) and focal length. Experimental works for verification and repeatability were conducted to determine the optimal drilling parameters.

3. Results and Discussion

TMV using laser drilling essentially encounters a difficulty to obtain the parameters for optimal process. The 1000 μm thickness compound was tested by two ultrafast picosecond lasers and thinner compounds, such as 100 μm thickness was applied to determine and verify the optimal parameters for removing fillers and 200 μm thickness was used for repeatability test. All DOE laser parameters can be found in ¹² .

3.1 Comparison of Green Laser and IR Laser

By comparison, the absorption rate of IR to epoxy resin is larger than Green to epoxy resin. Figure 4 demonstrates the depth of cut vs number of cutting path and pulse energy for IR laser and Green laser. An increasing in number of pass results in an increasing in the depth of cut. Similar case for pulse energy was observed. Focal position at entrance, middle and bottom impact the area of HAZ, the width of bottom trench and the depth of cut. Among these measured data, Green laser received less HAZ, less drop out fillers and fast removal of fillers. Therefore, Green laser was applied to through mold via process for all results shown in this paper. Figure 5 presents the tapered trench with counterbored entrance using Green laser drilling (via dimension: 1000 μm height, 190 μm width of entrance, 140 μm width of upper via and 110 μm width of bottom via). The solder bump on the top surface of substrate also exposed. For an interconnected PoP package, the counterbore entrance of a through via is designed for filling with conductive paste, while the countersink entrance is planned for sputtering on the side wall.

Figure 4

Comparison of Green and IR lasers (a) Depth of Cut vs. Number of Pass, (b) Depth of Cut vs. Pulse Energy

Figure 5

Tapered trench with counterbored entrance after Green laser drilling

3.2 Action for Trench Limitation

Figure 6 demonstrates microscopic photo and line profile using Keyence VK-X200 for the trench shown in Figure 5 . The shape of filler has been fabricated in round shape with diameter ranged from 25 μm to 35 μm for molding process. Parametric studies show that random fillers leakage is observed during fabrication and the minimum width of bottom trench should larger than triple filler size, e.g. φ110 μm for filler size of 35 μm, otherwise fillers are stuck on the trench bottom.

Figure 6

Microscopic photo and line profile of the cross-section trench after Green laser drilling

Variation of entrance consists of machine vision variation, xy-table variation and galvo variation. The aforementioned variations depend on apparatus. For fast removal of fillers, the minimum variation of the entrance should be double filler size, e.g. filler size 25 μm makes 50 μm variation in this study. Therefore, 190 μm minimum entrance width is required for a 140 μm upper via width and 110 μm bottom via width. For a straight via including variation, the minimum width of bottom is found to be quadruple of filler size. The size of spherical filler is 25 μm and the minimum through straight via (without counterbore) is drilled with 100 μm bottom width and 180 μm upper width.

Although all the experiments conducted with no gas-aided laser in this study, it should be noted that the imitation of the trench dimension depends not only the laser performance, but also blowing & suction system.

3.3 Filler Size and Spatial Distribution in EMC

3.3.1 Sidewall round Profile

Figure 7 (a) demonstrates round profile caused by different sizes of fillers. As can be seen, fillers with various sizes are mixed in a 100 μm thickness compound and result in a round profile after laser drilling.

Figure 7

High density of fillers (a) round profile caused by different size of fillers (b) smaller cutting angle for higher density (100 μm thickness compound)

3.3.2 High Density of Fillers

Figure 7 (b) presents high density of fillers in a 100 μm thickness compound. Higher density of fillers region tends to get the smaller cutting angle (1∼2°).

3.3.3 Low Density of Fillers

Figure 8 (a) indicates that lower density of fillers would be inclined to receive a larger cutting angle (7∼9°). For repeatability test, a thicker compound, e.g. 200 μm thickness compound was used to analyze failure modes on laser machined sidewall. Irregular profile caused by drop-out larger fillers was frequently observed in the low density region, which shown in Figure 2 (b).

Figure 8

Uneven spatial distribution of fillers (a) lower density of fillers receive larger cutting angle (b) uneven filler density with irregular profile (200 μm thickness compound)

3.3.4 Uneven Density

Figure 8 (b) displays the irregular profile on sidewall due to uneven filler density, e.g. lower density of fillers beneath surfaces and higher density of the fillers in the middle. This would receive the worst interconnection quality in the follow-up sputtering process.

3.3.5 Better Laser Drilled Profile

Figure 9 (a) , (b) exhibit that the smaller size filler would tend to obtain the smaller cutting angle for thickness of 100 μm and 200 mm, respectively. It should be noted that samples of filler size and spatial distribution were strictly controlled in the EMC. The optimal parameters for drilling of 100 μm thickness are 2.14W power, 55 kHz frequency, 200 μm/s cutting speed, 2 passes and 255 μm focal length. In the case of 200 μm thickness, the optimal laser drilling parameters are 26 A electrical current, 55 kHz frequency, 200 μm/s cutting speed, 5 passes and 255 μm focal length.

Figure 9

Better laser drilled profile on sidewall for (a) 100 μm (b) 200 μm thickness compound

Preliminary results revealed that repeatability on each case was very unsatisfactory and poor. An increase in the thickness of compound did not receive the better performance. Most of cases illustrated poor perpendicularity on the machined surface along the sidewall. This is because the size and spatial distribution of fillers have a great impact on edge quality on the drilled surface. Irregular and uneven sidewall of the through via would lead to poor reliability and low-grade interconnection on TMV PoP package.

4. Conclusions

In this paper, through mold via (TMV) using ultrafast picosecond lasers has been developed and the insight of fillers in EMC affected laser drilling parameters has been also explored. The main results are summarized as follows:

1. 1.

   Drilled trench performance depends on the material of filler in the molding compound. The results show that the filler particles with various sizes are not uniformly distributed within the EMC. These differences could be harmful to the reliability of the TMV PoP packages.

2. 2.

   The specification of the trench includes aspect ratio, minimum bottom trench width and variation. Minimum variation requires two times filler size and minimum bottom trench width should be four times of filler size. Minimum entrance width of counterbore, countersink and straight via should take into account the aspect ratio.

3. 3.

   Different laser will create different minimum trench size because the variation highly depends on laser equipment.

4. 4.

   The limitation of the trench dimension depends not only the laser performance, but also blowing and suction system.

5. 5.

   Fillers in EMC affects laser process. Optimal laser drilling parameters between fast removal of fillers and better edge quality on sidewall were determined.