Several industries such as microelectronic and optoelectronic device assembly, printed circuit board (PCB) manufacturing, and medical device manufacturing sectors extensively use surface preparation through the removal of surface contamination followed by surface activation through plasma processing.
At the time of surface preparation by plasma, the contaminants are first removed from the surface and this is followed by activating the surface for an array of applications such as promoting fluid flow and improving adhesion.
The highly reactive mixtures of gas species — plasmas — contain large concentrations of ions, electrons, free radicals, and other neutral species. Plasma is an established technology and offers an efficient, versatile, environment-friendly, and cost-effective method for altering the surface properties of materials. For both surface contamination removal and surface activation, plasma treatment can be used without having to change any bulk characteristics and without having to create any dangerous by-products.[1, 2]
Surface activation is a kind of process in which surface functional groups are replaced with different atoms or chemical groups from the plasma. Plasma source gases like argon, oxygen, hydrogen, or a combination of these gases are utilized to realize surface activation of materials.
Surface contamination removal through plasma processing is known to be an ablation process, wherein physical sputtering and chemical etching are mainly carried out. Organic contaminants like oxidation and epoxy residue, residual organic solvents, and mold release compounds present on the surface of several industrial materials are removed by the plasma process. Such surface contaminants usually experience repetitive chain scission under the influence of ions, electrons, and free radicals of the plasma until their molecular weight is low enough to volatilize in the vacuum.[2, 3]
In the case of microelectronic assembly applications, the surface contaminants contain metal oxides and organic substances that are introduced by bond grease at the time of manual handling, soldering process, overexposure to atmospheric air, oil fumes in the atmosphere, and photoresist utilized for photoprocessing substrates. During the manufacturing process, it is rather difficult to prevent surface contamination. By contrast, plasma processing eliminates the contaminants and also renders the surface active and clean.
This leads to improved wire bonds and reduced occurrence of delamination at the interface, given the fact that surface contamination is chiefly responsible for causing poor adhesion and wire bond pull strength.[4-6] Consequently, plasma processing has been widely employed for removing oxides, enhancing wire-bond strength, improving die attachment, eliminating delamination in microelectronic and optoelectronic sectors, and promoting void-free underfill.[2, 4–9]
XPS, AFM, SEM, and contact angle measurement are standard experimental techniques intended for assessing the surface property. Contact angle measurement — a simple and low-cost technique — can be used to evaluate the effectiveness of the surface contamination removal and surface activation processes.
Plasma treatment combined with the surface modification of plastics, metals, and ceramic surfaces boosts the wettability of those surfaces as established by the contact angle. In general, the surface energy is inversely proportional to the contact angle. Usually, this increased energy and decreased contact angle directly correlate with improved adhesion because organic contaminants have been removed at the time of the plasma treatment, and free radicals and the polar functional groups form on the surface, enabling a better interface between the surface and the normally polar fluid.
Figure 1 shows the correlation of the level of interfacial organic contamination as determined by XPS, in relation to the contact angle established on the copper leadframe for Ar and O2 plasma treatments. The data demonstrates that the level of organic contamination decreases proportionally when there is a decrease in the contact angle. The outcome clearly demonstrates that the contact angle measurements are indeed a good indication of the level of organic contamination existing on copper substrates.
Surface contamination removal and surface activation by plasma process can enhance the adhesion between the substrate and components, like diodes, die, thermoelectric coolers, and fiber. When both the die and substrate surfaces are clean, the die attach compound will adhere well to both the die and the substrate, and therefore, this approach is usually desired. Plasma cleaning prior to component attachment promotes better contact, better heat transfer, and minimal voiding.
Successful wire bonding is inhibited by the presence of organic contaminants and oxides on bond pads. Therefore, assurance of a surface free from oxides and organic contaminants is significant to achieve excellent bond yields. The data shown in Table 1 indicates the effect of argon plasma cleaning on wire bond yield.
The samples were initially plasma cleaned with argon for a period of 10 minutes using plasma conditions like 100 W and 0.2 Torr, and then, they were subjected to pull tests. The samples cleaned by plasma showed an average pull strength of 6.65 g with a typical deviation of 1.57, whereas the control showed an average pull strength of 5.3 g with a typical deviation of 1.89. The data indicate that the bonding strength has been enhanced after plasma cleaning.
||# of Devices
||# of Wires
||# of Bond Failures
The underfill process poses a unique challenge in flip chip packaging, particularly designs that employ high-density ball placement, large dies, and tight gaps. It has been demonstrated that plasma increases surface energy, minimizes voiding, boosts wicking speeds, and promotes adhesion.
As the plasma treatment time increases, the contact angle on the covered substrate surface and under the die decreases as illustrated in Figure 2. Figure 2 also shows the effect of die size; a larger dye will make it harder for the plasma to enter between the die and the substrate.
Figure 2. Surface contact angle underneath the die after plasma treatment with different plasma exposure time.
Encapsulation and Mold
The plastic encapsulant meant for semiconductor applications aims to provide adhesion to various package components, excellent corrosion and chemical resistance, adequate mechanical strength, high moisture resistance in the temperature range utilized, high thermal conductivity, and the corresponding coefficient of thermal expansion to the materials it interfaces with. In particular, the potential to form exceptional adhesion with package components and to stay bonded is extremely important. This is because delamination along the interfaces is a foremost reliability issue for plastic encapsulated microcircuits, or PEMs. The bond strength and adhesion are significantly enhanced through plasma treatment.
Figure 3 shows data demonstrating an increase in the bond strength by about a factor of two. The material utilized in this case was a PPS plastic shaped into a multi-pin connector. Epoxy cement (Abelbond #789-3) was used to bond cadmium and nickel wires into position and these were cured and the bonds were tested. Subsequently, plasma treatment was run in the March PX-500 system.
- Pressure: 180 mTorr
- Gas: Argon
- Power: 200 W
- Time: 15 minutes
The adhesion between the encapsulant and leadframe in PEMs was characterized using leadframe pull-out test, as shown in Figures 4a and 4b.
Figure 4a. Maximum de-bond load as a function of after plasma exposure time. The plasma condition: H2 (50%) and Ar (50%), 5 minutes. 234–300 mTorr, and 400 W.
Figure 4b. Maximum de-bond load as a function of surface contact angle. The plasma condition: H2 (50%) and Ar (50%), 5 minutes. 234–300 mTorr, and 400 W.
As the plasma exposure time increases, the maximum de-bond load decreases. The debond load is a measurement of the bond strength between the encapsulant and the leadframe. Larger bond translates to better adhesion. The association between the debond load and the contact angle is illustrated in Figure 4b. In general, the debond load is inversely proportional to the contact angle. Therefore , the contact angle measurement technique is an excellent indicator of bond strength in encapsulation processes.
Figure 5 shows the surface contact angle on the copper leadframe along with the plasma treatment time. The surface contact angle decreases as the plasma treatment time increases. The surface contact angle also depends on the plasma operating conditions, like time, pressure, gas selection, and power input. Figure 5 shows that reduced power affects the effectiveness of plasma treatment.
Marking is another area where plasma surface preparation is employed. The activated surface can enhance the adhesion of aqueous ink, whereas the surface prepared by plasma enhances the adhesion of aqueous-based inks.
Plasma technology can be employed for preparing the surface prior to hermetic sealing of a laser diode device. A plasma-cleaned surface improves the adhesion at the interface, allowing for a more reliable weld.
Plasma conditions are essential for the plasma surface activation as well as contamination removal. Gases, input power, operating pressure, plasma exposure time, sample location in the chamber, and electrode configuration are the main factors of the plasma process. All the parameters should be carefully established for various applications. Argon plasma process is a physical process and therefore a lower operating pressure has to be chiefly applied. However, a higher operating pressure is needed in reactive gas plasma, including oxygen, because chemical reaction happens to be dominant on the surface.
One frequently asked question is, “How long does a surface remain active?” The reason is the activated surface is susceptible to the environment. Generally, the activated surface will gradually lose its wettability owing to storage contamination, self-contamination, or air contamination. Figure 6 shows one such instance, where the same PPS plastic and plasma treatment conditions are employed. The change in contact angle as a function of time is shown by the data. Due to the surface recontamination illustrated in Figure 6, the adhesion strength will reduce with the increase in exposure time following plasma treatment.
Storing the treated samples is another issue that emerges. In this study, the same kinds of PPS plastic samples were subjected to plasma treatment and subsequently placed in a Teflon FEP bag, a polyethylene bag, or covered in a plasma-treated aluminum foil, as illustrated in Figure 7. The surface activation of all the other samples degrades eventually, except for the samples kept in FEP.
At Nordson March, the technical personnel will be happy to share their experience in plasma technology for specific applications of customers. They would be equally happy to publish any data that customers would like to share with others in the domain.
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- 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.
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This information has been sourced, reviewed and adapted from materials provided by Nordson MARCH.
For more information on this source, please visit Nordson MARCH.