Epoxy-only and epoxy-acrylic underfill formulations containing key reworkable resin
Abstract
A highly reliable and reworkable underfill adhesive based on thermoset epoxy resin possessing thermally reversible dicyclopentadiene (DCPD) moiety is described. The adhesive can be cured rapidly at moderate temperatures resulting in high Tg cured network, which gives high reliability to the bonded semiconductor components. The inherent thermal reversibility of DCPD moiety causes network breakdown at high temperatures enabling easy removal of defective semiconductor chips. A discernible trend between loading level of the thermally reversible epoxy resin and high-temperature die shear strength was observed. Using this novel adhesive system, both high reliability and reworkability can be achieved concurrently, which is normally not possible with other thermoset adhesive systems. The epoxy resin used in the study was scaled up to multi-kg quantities demonstrating industrial applicability of the approach.
Keywords
- epoxy
- dicyclopentadienedicarboxylic acid diglycidyl ester
- thermoset
- reworkable
- underfill adhesive
- thermally reversible
1. Introduction
The popularity of handheld display devices (HHDDs) has made their demand increase dramatically in recent years. Manufacturing throughput has consequently been challenged to meet the growing demand. One area that is particularly troublesome for manufacturers is the treatment and handling of defective semiconductor chips on a circuit board. For instance, during the manufacture of a circuit board subassembly, a multitude of semiconductor devices are electrically connected to the circuit board in chip scale packages (“CSPs”), ball grid arrays (“BGAs”), land grid arrays (“LGAs”), and the like [1]. The board may then be tested to evaluate function and sometimes the board fails. In such cases, it is desirable to identify the semiconductor device that caused the failure, remove it from the board, and reuse the board with the remaining functioning semiconductor devices. This would save cost for the manufacturer of HHDDs.
Ordinarily, semiconductor devices (also called as chips) are connected to electrical conductors such as Cu pads on printed circuit boards (PCBs) by solder connection as shown schematically in Figure 1A. The coefficient of thermal expansion (CTE) of the Si die and the solder balls is significantly different. When the resulting subassembly in a HHDD is exposed to mechanical shocks such as vibration, distortion, drop (Figure 1C), or rapid temperature change (thermal shock, for example, when a device is left in a car during winter or summer), the reliability of the solder connection between the circuit board and the chip often becomes suspect. There are other modes of failure that can occur in a subassembly such as electrical shorting or stress cracks in solder balls (B & D, respectively, Figure 1).
Underfill adhesives are widely used to improve the overall thermal shock resistance, mechanical and electrical reliability of the assembly [2]. After a chip is mounted on a circuit board, the space between the chip and the PCB is filled with an underfill adhesive resin. Once the adhesive is cured, the stress is uniformly distributed throughout the bondline rather than just at the contact point between the solder ball and the chip. This enhances the overall thermal and mechanical shock properties and thus the reliability of the assembly [3, 4]. The adhesive formulation is typically a low-viscosity liquid (<1000cPs), which penetrates the gap between the chip and the PCB by capillary action when dispensed. The circuit board can also be slightly heated to about 50°C to lower the viscosity further and to accelerate the capillary fill process. In a high-throughput assembly, the dispensing and the fill process is completed in less than a minute, which requires low viscosity in the underfill resin. The adhesive resin is typically cured in the temperature range 120–130°C in less than 10 minutes. A picture of two CSPs with cured underfill adhesive is shown in Figure 2 (adhesive is shown in black color indicated with arrows).
The underfill adhesives are typically thermosetting resin compositions that form cross-linked networks when cured. With conventional thermoset adhesives, it is difficult to remove the chip without damaging the subassembly in the event of a failure of a semiconductor chip on the circuit board. Several approaches have been published in the literature on reworkable, reversible, or degradable thermoset networks used in a number of adhesive applications [5, 6, 7, 8, 9]. Reworkability or removability is also a valuable attribute of adhesives used for electronic packaging applications including underfill adhesives [10]. It is highly desirable for an underfill adhesive to provide good electrical reliability, mechanical and thermal shock resistance, while allowing for the semiconductor chips to be easily separated in a defective assembly without causing damage to the circuit board. Several chemistry approaches have been explored to make the underfill adhesive reworkable at high temperature [11, 12]. For good reliability (mechanical and thermal shock resistance), high modulus and Tg are essential for underfill adhesives, which allow them to have lower CTE at the service temperature of the HHDDs or during T-cycle tests (hence reduced mismatch in CTEs between bonded substrates) since the CTE of thermosets increases rapidly above the Tg. However, with conventional thermosets, it is difficult to achieve both high reliability and reworkability without a built-in rework chemistry mechanism. The results shown below demonstrate that by using a carefully designed thermoset resin system, both high reliability and reworkability can be achieved concurrently.
2. Results and discussion
The concept of highly reliable and reworkable thermoset underfill adhesive system is shown schematically in Figure 3. The adhesive needs to be a highly cross-linked system (high modulus and Tg) for high reliability at the service temperature of the HHDD or during thermal cycle (T-cycle) tests. The thermoset adhesive needs to undergo network break down at rework temperatures (typically around 220°C for this application) for easy removability of the faulty semiconductor chip. The design needs to be such that the rework temperature is sufficiently high so that there is no network breakdown occurring during adhesive curing or thermal cycling reliability tests. Typically for T-cycle tests, the bonded components are subjected to a temperature ramp from −40°C to 80°C (thermal shock) with a 15-minute hold time at each temperature. For good reliability performance, 1000 T-cycles without electrical failure are required, and hence, the need for a high-performance underfill adhesive system.
2.1 Resin system design
For the resin system design, the built-in reworkable or reversible chemistry was carefully chosen so that the cross-linked polymer network breakdown takes place at sufficiently higher temperature than that used for adhesive curing. The dicyclopentadiene backbone was chosen because of its high Tg and relatively high retro-Diels-Alder temperature. Based on previous literature report on activation energy required for retro reaction [13], we sought to incorporate electron withdrawing ester groups on dicyclopentadiene double bonds such that the network breakdown occurs at the correct temperature range for good reworkability. Thiele’s acid
For the proof-of-concept study, the diacid
Both
To investigate if facile uncatalyzed homopolymerization of epoxy functionality in
2.2 Development of underfill formulations
Epoxy resin
Materials | Formula function | Epoxy-only formulas | Epoxy-acrylic formulas | |||
---|---|---|---|---|---|---|
F1 | F2 | F3 | F4 | F5 | ||
Bisphenol A epoxy | Resin | 41.4 | 42.3 | 43.9 | 45.4 | |
Bisphenol F epoxy | 10.6 | 11.7 | 13.1 | 14.6 | 10.0 | |
10.0 | 8.0 | 5.0 | 2.0 | 5.0 | ||
Naphthyl 1,6-diglycidyl ether | 20.0 | |||||
Trifunctional epoxy | 5.0 | |||||
Tricyclodecane dimethanol diacrylate | 9.9 | |||||
Isobornyl methacrylate | 5.0 | |||||
Bisphenol E cyanate ester | 5.0 | |||||
p-tert-Butylphenyl glycidyl ether | Diluent | 15.3 | 15.3 | 15.3 | 15.3 | 6.0 |
Epoxy-imidazole adduct | Hardener | 16.7 | 16.7 | 16.7 | 16.7 | 20 |
Cationic polymer | Dispersant | 0.7 | 0.7 | 0.7 | 0.7 | |
Ethyl/ethyl-hexyl acrylate copolymer | 0.4 | |||||
Epoxy silane | Adhesion Promoter | 0.7 | 0.7 | 0.7 | 0.7 | |
Carbon black in epoxy | Color | 0.9 | 0.9 | 0.9 | 0.9 | 5 |
Dicyandiamide | Hardener | 3.6 | 3.6 | 3.6 | 3.6 | |
Barbituric acid | Inhibitor | 0.1 | 0.1 | 0.1 | 0.1 | |
Butylated hydroxytoluene (BHT) | 0.1 | |||||
Glycidyl methacrylate | Crosslinker | 8 | ||||
tert-Butyl peroxy-2-ethylhexanoate | Radical initiator | 0.6 | ||||
Total | 100 | 100 | 100 | 100 | 100 |
For good mechanical and thermal shock resistance, the underfill adhesive needs to exhibit a stable storage modulus in the service temperature and T-cycle test temperature range (
The DMA storage modulus plot for formula F-4 with the lowest level of resin
Table 2 shows comparison of Tg and storage modulus numbers (MPa) at different temperatures (
Formula | E′(MPa) (−75°C) | E′(MPa) (25°C) | E′(MPa) (85°C) | E′(MPa) (125°C) | E′(MPa) (220°C) | Tg (tanδ) |
---|---|---|---|---|---|---|
F1 | 2757 | 2159 | 1871 | 662 | 11.6 | 140 |
F2 | 3069 | 2322 | 2024 | 994 | 16.5 | 143 |
F3 | 2713 | 2092 | 1869 | 1188 | 22.4 | 145 |
F4 | 2627 | 1986 | 1807 | 1326 | 33.7 | 148 |
F5 | 3241 | 2507 | 2005 | 809 | 40.6 | 147 |
UF3800 | 2459 | 1860 | 1110 | 185 | 57 | 124 |
UF3808 | 3320 | 2502 | 2059 | 945 | 106 | 145 |
UF3810 | 4160 | 3031 | 1369 | 451 | 72 | 145 |
E1216M | 3750 | 2858 | 2653 | 1452 | 88 | 144 |
2.3 Die shear adhesion properties
An important property of underfill adhesives is good die shear adhesion at room temperature (25°C) and the peak temperature experienced during an assembly reflow process (260°C). In contrast, reworkable underfill adhesives need to exhibit high die shear adhesion at room temperature for high reliability performance while that measured at higher temperature needs to be low enough for easy removal of faulty chip at the rework temperature (220°C), yet high enough to prevent any solder extrusion during a reflow step (< 1–2 min @ 260°C). The die shear tests were performed on Dage 4000 instrument from Dage Precision Industries following MIL-STD-883 2019.9 die shear method. The 25°C die shear tests were completed using 3 mm2 size SiN dies on bismaleimide-triazine (BT) substrate. Adhesion at 260°C using the same 3 mm2 size dies was too low among all underfills for direct comparison. Therefore, 260°C die shear tests were performed using 7.6 mm2 size dies for better response and comparison of underfill adhesion. The die shear adhesion of reworkable formulas F1–F5 was measured and compared with controls.
Figure 9 shows the comparison of die shear strengths at room temperature. Formulas F1–F5 show similar or superior die shear strength than the highly reliable underfill benchmark UF3808 while another control formula UF3800 shows the lowest strength among formulas tested. There appears to be no discernible trend between loading level of resin
The die shear tests performed at 260°C further corroborate the storage modulus results shown in Figures 7 and 8. The 260°C die shear strengths for formulas F1–F5 and their comparison with benchmarks are shown in Figure 10. The shear strengths correlate well with the amount of reworkable resin
2.4 Reworkability study
Select experimental underfill formulas F1 (contains 10% of
Table 3 shows qualitative evaluation guideline and score card for the board rework process. The rating of the rework process considers a multitude of factors such as ease of die removal, amount of underfill left on the board after cleaning, number of pads and traces damaged on the board, total cleaning time, and solder mask damage, each with its own weightage (total adds up to 1). The ease of rework is rated on a scale of 1–10 where a rating of 1 indicates poor reworkability and 10 best reworkability.
Weight | Score | 1 | 3 | 5 | 7 | 10 |
---|---|---|---|---|---|---|
0.3 | Ease of removal of die | Suction+high force, cannot be removed | Suction+high force to turn & remove | Suction+medium force to turn & remove | Suction+low force to turn & remove | Removes on suction |
0.2 | Amount of UF left after cleaning | >75% | 50–75% | 25–50% | 0–25% | 0% |
0.1 | # of pads damaged | >9 | 6–9 | 3–6 | 1–2 | 0 |
0.1 | # of traces damaged | >9 | 6–9 | 3–6 | 1–2 | 0 |
0.2 | Time to clean | >150s | 120–150s | 90–120s | 45–90s | <45s |
0.1 | Solder mask damage | Significant damage (>15% area) | Damage 10–15% area | Damage 5–10% area | Damage to <5% area | No damage |
Benchmark underfill formulas UF3800, UF3808, and E1216M were compared with F1, F2, and F5 formulas for reworkability performance. A test board similar to that shown in Figure 12 was used to bond the chips using benchmarks and the experimental formulas. Table 4 shows total reworkability score for these formulas involving a multitude of factors discussed before. As expected, UF 3800 showed a high rating of 8 while the other two benchmark formulas UF3808 and E1216 formulas fared poorer for reworkability. The reworkability score for formulas F1, F4, and F5 was similar to or better than UF3800. The hybrid epoxy-acrylic formula F5 showed slightly inferior reworkability score than F1 and F2. The likely reason for relatively lower rework score for F5 as compared with F1 and F4 was discussed in the previous section and is suspected to be from the norbornene double bond copolymerization, which would result in partially (depending on extent of copolymerization) non-cleavable networks.
Formulas | UF3800 | UF3808 | E1216M | F1 | F4 | F5 |
---|---|---|---|---|---|---|
Resin | 0 | 0 | 0 | 10 | 2 | 5 |
Amount of UF left1 | 1.33 | 0.87 | 0.93 | 1.07 | 1.4 | 1.13 |
Ease of Removal2 | 1.95 | 1.55 | 1.50 | 2.70 | 2.10 | 1.95 |
# Pads Damaged3 | 0.85 | 1.00 | 0.90 | 0.92 | 1.0 | 1.0 |
# Traces Damaged4 | 0.95 | 1.00 | 0.90 | 0.95 | 1.0 | 1.0 |
Time to clean5 | 1.90 | 0.60 | 1.70 | 2.00 | 2.0 | 1.70 |
Solder mask damage6 | 1.0 | 0.62 | 0.5 | 0.92 | 1.0 | 0.95 |
Final rework score | 8.0 | 5.6 | 6.4 | 8.6 | 8.5 | 7.7 |
Figure 14 shows pictures of the test board after removal of the bonded die for the control formulas UF3800 and UF3808. The images on the left side show the substrate after removal of the die before rework and cleaning while those on the right show substrate board after cleaning. As expected, UF 3800 results in a clean board after die removal, rework, and cleaning. In contrast, UF 3808 causes damage to the board during die removal and leaves lots of adhesive residue even after cleaning. Figure 15 shows images of the substrate board after die removal (on the left side) and after the rework process and cleaning (on the right) for formulas F1, F4, and F5. Consistent with the storage modulus profile and high-temperature die shear results discussed previously, all of these formulas enable easy rework process that is similar to or slightly better than reworkable UF3800 benchmark as evidenced by the clean substrate board obtained after the rework process. This result further demonstrates that resin
2.5 T-cycle reliability tests
Since the HHDDs with bonded semiconductor components can be subjected to thermal shocks as described previously, the bonded components were tested for T-cycle reliability using an underfill board array containing 15 bonded semiconductor components similar to that shown in Figure 12. The reliability test was performed by using a 30-minute temperature ramp from
Formula | 200 cycles | 400 cycles | 600 cycles | 800 cycles | 1000 cycles | 1200 cycles | 2000 cycles |
---|---|---|---|---|---|---|---|
UF3800 | 0/15 | 0/15 | 3/15 | 7/15 | 9/15 | 10/15 | 15/15 |
UF3808 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 |
E1216M | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 |
F1 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 |
F4 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 |
F5 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 | 0/15 |
3. Conclusions
Synthesis of a new diglycidyl ester epoxy resin possessing dicyclopentadiene backbone was described. The resin was formulated in underfill adhesive formulations to provide cured adhesives with high Tg and modulus. Clear dependence of loading level of thermally reversible resin
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