Improving Adhesion with Surface Preparation

Surface preparation through surface contamination removal and surface activation, by plasma processing is extensively employed in a number of industries such as printed circuit board (PCB) manufacturing, optoelectronic and microelectronic device assembly, and medical device manufacturing industries. During surface preparation by plasma, the contaminants are removed from the surface and the surface is activated for a wide range of applications including enhancing adhesion and promoting fluid flow.

Plasmas – the highly reactive mixtures of gas species – consist of large concentrations of free radicals, electrons, ions and other neutral species. As a proven technology, plasma provides a versatile, efficient, economic and environmentally friendly means for modifying the materials’ surface properties. Plasma treatment can be employed for surface activation and contamination removal without changing any bulk properties and without producing any hazardous by-products [1, 2].

Surface activation is a type of process where surface functional groups are substituted with different chemical groups or atoms from the plasma. Plasma source gases such as hydrogen, oxygen, argon or a mixture of these gases are used to achieve surface activation of materials [2].

Surface contamination removal by plasma processing is an ablation process in which chemical etching and physical sputtering are mainly performed. The plasma process eliminates organic contaminants such as residual organic solvents mold release compounds, oxidation and epoxy residue present on the surface of many industrial materials. These surface contaminants typically undergo repetitive chain scission under the effect of free radicals, electrons and ions of the plasma until their molecular weight is low enough to volatilize in the vacuum [2, 3].

In microelectronic assembly applications, the surface contaminants consist of organic substances and metal oxides that are introduced by bond grease during manual handling, overexposure to atmospheric air, photoresist used for photoprocessing substrates, soldering process and oil fumes in the atmosphere. It is difficult to prevent surface contamination during the manufacturing process. Plasma processing not only removes the contaminants but also makes the surface clean and active. This results in enhanced wire bonds and decreased occurrence of delamination at the interface considering the fact that surface contamination is a main cause of poor wire bond pull strength and adhesion [4-6]. As a result, plasma processing has been extensively used for removing oxides, enhancing die attachment, improving wire-bond strength, promoting void-free underfill, and eliminating delamination in optoelectronic and microelectronic industries [2. 4-9].

Experimental Methods

SEM, AFM, XPS and contact angle measurement are general experimental methods meant for surface property evaluation. Contact angle measurement is a simple, economical method for assessing the effectiveness of the surface activation and surface contamination removal processes.

Plasma treatment and surface modification of ceramic, metals and plastics surfaces increases the wettability of those surfaces as determined by the contact angle. Generally, if the surface energy is higher, the contact angle will be lower. This increase of energy and decrease of contact angle often correlates directly with better adhesion, because organic contaminants have been eliminated during the plasma treatment and the polar function groups and free radicals form on the surface enabling a better interface between the typically polar fluid and the surface.

Shown in Figure 1 is the correlation of the level of interfacial organic contamination as established by XPS, relative to the contact angle determined on the copper leadframe for O2 and Ar plasma treatments.

The data shows that as the contact angle decreases, the level of organic contamination also decreases proportionally. The result distinctly shows that the contact angle measurements are definitely a good indication of the level of organic contamination present on copper substrates.

Experimental Methods

Figure 1.

Applications

Component Attach

Surface activation and contamination removal by plasma process can improve the adhesion between the substrate and components, such as die, diodes, fiber and thermoelectric coolers. A clean die and substrate surface promotes better adhesion of the die attach compound to both the substrate and the die, and hence it is generally preferred. Plasma cleaning before component attachment provides better heat transfer, better contact and minimal voiding.

Wire Bonding

The presence of oxides and organic contaminants on bond pads inhibits successful wire bonding. Therefore, assurance of an oxide-and-contaminant-free surface is important to obtain good bond yields. Table 1 shows data, which indicates the impact of argon plasma cleaning on wire bond yield.

The samples were plasma cleaned with argon for 10 minutes using the following plasma condition: 100 watts and 0.2 Torr, and were then subjected to pull tests. The plasma-cleaned samples demonstrated an average pull strength of 6.65 grams with a standard deviation of 1.57, while the control revealed an average pull strength of 5.3 grams with a standard deviation of 1.89. The data shows that the bonding strength has been improved following plasma cleaning.

# of Devices # of Wires Wire Size
(mils)
Pull Test # of Bond Failures Failure Rate
Lab #1
Plasma Cleaned 25 1380 1.5 5 g 6 0.43%
Plasma Cleaned 100 1.0 3 g 11 11%
Control 25 1378 1.5 5 g 10 0.73%
Control 94 1.0 3 g 23 24.5%
Lab #2
Plasma Cleaned 50 1375 3.5 g 8 0.58%
Control 50 1375 3.5 g 26 1.89%
Lab #3
Plasma Cleaned 10 840 1 0.12%
Control 10 840 29 3.45%

Flip Chip

A unique challenge in flip chip packaging is the underfill process, especially designs that use tight gaps, large dies and high-density ball placement. Plasma has been shown to boost surface energy, minimizing voiding, promoting adhesion and increasing wicking speeds.

The contact angle under the die and on the covered substrate surface decreases with the increasing plasma treatment time as shown in Figure 2. Also shown in Figure 2 is the impact of die size; if the dye is larger, it would be more difficult for the plasma to penetrate between the substrate and the die.

Flip Chip

Figure 2. Surface contact angle under the beneath of die after plasma treatment with different plasma exposure time.

Encapsulation and Mold

The aim of the plastic encapsulant for semiconductor applications is to provide adhesion to different package components, sufficient mechanical strength, excellent chemical resistance and corrosion, high thermal conductivity, matched coefficient of thermal expansion to the materials it interfaces with, and high moisture resistance in the temperature range used. Particularly, the ability to form excellent adhesion with package components and to remain bonded is very crucial, because delamination along the interfaces is a major reliability problem for plastic encapsulated microcircuits (PEMs). This adhesion and bond strength is considerably improved by plasma treatment.

The data shown in Figure 3 shows about a factor of two increase in the bond strength. In this case, the material used was a PPS plastic molded into a multi-pin connector. Nickel and cadmium wires were bonded into position by epoxy cement (Abelbond #789-3), cured and the bonds tested. Plasma treatment was run in the March PX-500 system.

  • Power: 200 watts
  • Gas: Argon
  • Pressure: 180 mTorr
  • Time: 15 minutes

Encapsulation and Mold

Figure 3.

Leadframe pull-out test is used to characterize the adhesion between leadframe and encapsulant in plastic encapsulated microcircuits, as shown in Figure 4 [4]. The maximum de-bond load decreases with increasing plasma exposure time. The debond load is a measure of the bond strength between the leadframe and the encapsulant. If the bond is larger, the adhesion is better. The relationship between the contact angle and the debond load is shown in Figure 4b. Generally, the debond load increases with decreasing contact angle. Therefore, the contact angle measurement method is a good indicator of bond strength in encapsulation procedures.

Maximum de-bond load

Figure 4. Maximum de-bond load as a function of after plasma exposure time (a) and surface contact angle (b). The plasma condition: H2 (50%) and Ar (50%), 5 min. 234-300 mTorr, and 400 W. [4]

The plasma condition

Figure 4b.

Shown in Figure 5 is the surface contact angle on copper leadframe with the plasma treatment time. As the plasma treatment time increases, the surface contact angle decreases. In addition, the surface contact angle relies on the plasma operating conditions, such as power input, gas selection, time and pressure. The figure shows that the effectiveness of plasma treatment is affected when power is reduced.

surface contact angle on copper leadframe

Figure 5.

Marking

Plasma surface preparation is also used in marking. The activated surface can improve the adhesion of aqueous ink, while the plasma prepared surface improves the adhesion of aqueous-based inks.

Hermetic Sealing

Plasma technology can be used to prepare the surface before hermetic sealing of laser diode device. A surface cleaned by plasma enhances the adhesion at the interface enabling a more dependable weld.

Considerations

Plasma Conditions

Plasma conditions are integral for the plasma surface activation and contamination removal. The major factors of plasma process include input power, gases, plasma exposure time, operating pressure, electrode configuration and location of the sample in the chamber. It is important that all the parameters are carefully determined for different applications. Since argon plasma process is a physical process, a lower operating pressure should be mainly applied. Yet, a higher operating pressure is required in oxygen or other reactive gas plasma because chemical reaction is dominant on the surface.

Life Time

The question that is generally asked is, "How long does a surface remain active?" This is because the activated surface is sensitive to the environment. In general, the activated surface will slowly lose its wettability due to self-contamination, air contamination and storage contamination. One such example is shown in Figure 6, where the same PPS plastic and plasma treatment conditions are used. The data shows the change in contact angle as a function of time. Owing to the surface recontamination shown in Figure 6, the adhesion strength will decline with increasing exposure time after plasma treatment.

Life Time

Figure 6.

Storage

Another problem that emerges is how to store the treated samples. In this analysis, the same PPS plastic samples were treated with plasma and then placed in a polyethylene bag, a Teflon FEP bag, or wrapped in a plasma treated aluminum foil, as shown in Figure 7. Except for the samples stored in FEP, the surface activation of all the other samples degrades with time

Storage

Figure 7.

Applications Laboratory

At March, the technical personnel are happy to provide their experience in plasma technology for customers’ applications. They would also be happy to publish any data customers would wish to share with others in the field. Customers are invited to direct their calls and faxes to March Plasma Systems, Attention: Applications Laboratory.

Reference

1. Handbook of Plasma Processing Technology, Fundamentals, Etching, Deposition, and Surface Interactions, Edited by S. M. Rossnagel, J. J. Cuomo and W. D. Westwood, Noyes Publication, Westwood, NJ, USA.

2. E. Finson, S. L. Kaplan, and L. Wood, Plasma Treatment of Webs and Films, Society of Vacuum Coaters, 38th Annual Technical Conference Proceedings (1995)

3. H. Yasuda, Plasma Polymerization, Academic Press, Orlaando, FL, USA, 1985.

4. Y. Sung, J. -K. Kim, C Y. Yue, and J. -H Hsieh. Bonding strengths at plastic encapsulant-gold-plated copper leadframe interface, Microelectronics Reliability 40 (2000) 1207-1214.

5. L. Wood, C. Fairfield, and K. Wang, Plasma Cleaning of Chip Scale Packages for Improvement of Wire Bond Strength, TAP technology, Second edition, 75-78.

6. F. Djennas, E. Prack, Y. Matsuda, Investigation of Plasma Effects on Plastic Packages delamination and Cracking, IEEE Trans CHMT 16 (1993) 919-924

7. L. J. Matienzo and F. D. Egutto, Adhesion Issues in Electronic Packaging, Solid State Technology, July (1995), 99-106.

8. R. N. Booth and P. E. Ongley, Plasma Treatment in Hybrid and Conventional Electronic Assemblies, Hybrid Circuits, 7 (1995).

9. H. K. Kim, Plasma Cleaning of Spacecraft Hybrid Microcircuits and Its Effect on Electronic Components, 1989 The International Society for Hybrid Microelectronics, 144-148.

Nordson MARCH

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