Plasma Treatment in Flip Chip Packaging

Flip chip packaging technology — thanks to its heat dissipation properties, I/O density, speed, and high performance — has become an advanced and highly demanded packaging method. The underfill process has a vital role to play because it can significantly increase the reliability and yield in flip chip assembly.

A process like that can lower the relative displacement between the die and substrate, thereby decreasing the stress related to the solder interconnection that takes place from the mechanical loading and thermal cycling of the structure. In addition, the process can safeguard the device from environmental hazards, thus improving the mechanical dependability of flip chip packages.

Conversely, the underfill process is fraught with several challenges, like slow wicking speed, potential delamination, and lower and unbalanced fillet height. While slow wicking speed increases product cost and reduces production throughput, lower and unbalanced underfill height will not only reduce the resistance of a flip chip device to delamination but will also reduce its tolerance to thermal and mechanical shock. Due to these factors, the solder balls are forced to withstand most of the deformation of the package assembly all through the cycling loading process, eventually resulting in premature shear failure.

Such challenges are associated with the properties of the underfill fluid, the flip chip device, and the substrate, like gap, bump density, size of the die, surface property of the substrate, and different die passivation materials. Despite specifying the geometries, underfill fluids, as well as the packaging materials, underfill performance only depends on the surface properties of the substrate and the die.

To achieve an effective underfill process in flip chip packaging, plasma surfaces should be modified. Such modifications are performed to enhance both surface adhesion and surface bondability and also tailor surface energy.

The plasma process is employed in several applications, such as increasing the uniformity and height of fillet, improving the underfill adhesion of flip chip devices and the pull strength of wire bonds, and altering the surfaces for better adhesion in encapsulation, lamination, and mold processes.

Plasma Technology

The partially ionized gas — plasma — consists of an electrically neutral mixture of physically and chemically active gas phase species, like free radicals, neutral particles, ions, electrons, and photons. Physical work can be done by the molecular species through sputtering, and chemical work can be done by the reactive radical species through a chemical reaction. Therefore, plasma has the ability to carry out numerous surface modification processes, like surface activation, crosslinking, removal of contamination, and etching by chemical reaction and physical bombardment.

One standard example of surface cleaning through physical bombardment is the argon plasma process. Oxygen plasma can do both physical and chemical work on surfaces because of the presence of ions and free radicals. Either downstream mode or direct mode can be used in plasma applications. In the direct mode, the substrate is directly exposed in the glow discharge zone. Considering that this process involves ions as well as free radicals, direct plasma has been demonstrated to be effective, aggressive, and quick; however, it also tends to impact specific devices that are susceptible to UV light and/or charge damage, and also to photon emission.

In the downstream mode, including ion-free plasma (IFP), the substrate is placed beyond the plasma glow discharge zone, usually downstream of the gas flow. It is said that the downstream plasma process is a mild process because the majority of ions and UV light are filtered much before the activated species are able to reach the substrate surface. Devices that are sensitive to UV exposure and ion bombardment can be subjected to this process. The downstream plasma process is slower compared to the direct plasma process. On a hypothetical basis, the IFP process is assumed to be the pure chemical process because it is the free radicals that mostly play a role in surface modification.

Review of Previous Work

Plasma for Surface Modification Beneath the Die

A previous study has demonstrated that plasma active species have the ability to enter the gap between the die and substrate and activate the surface under the die. This surface activation is important for utilizing plasma for underfill process. The experimental results indicate that the surface wettability under the flip chip die depends on plasma chemistry, the flip chip geometry, and the flip chip package materials. The chip’s size has an impact on the effectiveness of surface cleaning in flip chip packaging.

Upon reducing the die size, the surface contact angle placed on the center of the die and substrate also reduces. This implies that as die size decreases, the effectiveness of the plasma cleaning process increases. After plasma treatment, the surface material also has an impact on the surface contact angle. Under the same plasma condition, it was seen that the surface contact angle situated on the die surface is relatively lower than the surface of the substrate.

Further experimental results indicated that the surface contact angle — at the center of the die as well as on the substrate beneath the flip chip — depends on the plasma source gas. The oxygen-based plasma, when compared to nitrogen and argon, has a greater impact on the contact angle. O2 plasma > N2 plasma > Ar plasma is the trend of the contact angle.

Lower contact angle or higher surface energy can be obtained through oxygen plasma treatment. Within the same plasma conditions, the surface contact angles on the substrate and die reduce with increased oxygen ratio in O2/Ar and O2/N2 gas mixtures. In addition, plasma treatment also changes the surface composition under the die through a passivation layer of polyimide. When the oxygen content of polyimide is compared with untreated and treated samples, the oxygen composition on the die surface increases roughly 36% post the oxygen IFP plasma treatment.

Investigation of the surface functional group indicated that the overall oxy-functional group increases by about 19%. Such an increase in the oxy-functional groups on the surface of the die can chemically join the underfill materials at the time of the underfill process. Greater concentration of oxy-functional groups can lower the possibility of delamination. This interface delamination can be lowered by removing contaminants from the interfaces and then chemically activating the surface.

Plasma for the Underfill Process

During the underfill dispensing process, the high performance includes high wicking speed, high fillet, and uniform fillet height. Generally, it is desired that the underfill fluid dispenses quickly and, at the same time, creates a sufficient fillet height with remarkable uniformity. The wicking speed of the underfill is inversely proportional to the flow-out time, which is consecutively proportional to the contact angle’s cosine:

T=3µL2/hγcosθ.

Where T is the flow-out time in seconds, µ is fluid viscosity, L is flow distance, h is bump or bump height, γ is surface tension of liquid-vapor interface, and θ is the contact and wetting angle.

In certain flip chip packages, if the contact angle is lower, the wicking speed will be faster. Therefore, faster wicking speed translates to higher manufacturing line throughput. One example from the previous work showed that the contact angle on the center of the die was 40° and 20° before and after plasma treatment, respectively. The reduced contact angle transformed into a reduced flow-out time from 60 to 22 seconds — which is a definite improvement of 270%. This implies that when plasma is used in the underfill dispensing process of flip chip packaging, the production throughput improves significantly.

In general, a more uniform fillet height results in a more reliably packaged device, while an uneven fillet height leads to irregular stresses on the chip. This may ultimately lead to fillet cracking and package failure. The low surface energies of the substrate and die affect the uniformity and height of the fillet. The previous experiment indicated that after the oxygen IFP plasma treatment, the height of the fillet on the opposite side improved from 12.4% to 27.70% and imbalance of the fillet height reduced from 44.2% to 12.60%. This satisfies the parameter of fillet height and uniformity in the flip chip packaging sectors.

New Experimental Data and Discussion

Plasma for Underfill Dispensing Performance

In order to show how plasma improves the underfill dispensing performance, three samples — A, B, and C — with different die sizes and surface finishes were considered for assessment. The results are shown in Table 1. Based on these data, it can be seen that wicking speed is improved by plasma treatment and this can be observed by the reduced flow-out time during the duration of the underfill dispensing process.

Table 1. All samples were treated in an AP-1000, batch plasma machine from Nordson MARCH

  Sample Information
Sample A Sample B Sample C
Die Size 5 x 5 mm 5 x 5 mm 7 x 7 mm
Surface Finishes Cu/Ni/Au Cu/OSP Cu/Ni/Au
Pitch Between Joints 200 µm 200 µm 12.5 µm
Joint 100 µm 100 µm 350 µm
Plasma Parameters
O2 100 sccm 400 W 200 m Torr 3 min
Result
  Time to Flow-Out (sec)
Average St. dev Improvement
Sample A Untreated 9.07 1.61 17%
Sample A Treated 7.56 1.58 17%
Sample B Untreated 9.64 1.34 11%
Sample B Treated 8.6 1.77 11%
Sample C Untreated 24.23 4.15 37%
Sample C Treated 15.26 1.75 37%

 

This result corresponds with the previous results; yet, the scale of improvement was lower due to the variations in materials. In particular, plasma treatment boosts the surface energy, allowing better flow and wicking speed. This increased wicking speed depends on the surface finish and the die size. While the gold-finished flip chip shows improved wicking speed after plasma treatment, the flip chip with larger die size displays relatively more improvement.

X-ray images did not reveal any distinct difference between plasma-treated substrates and untreated substrates; however, the variation of underfill performance can be observed after the underfill dispensing process is completed. The comparison of optical images between plasma-treated and untreated flip chips after the underfill curing process is shown in Figure 1.

Top view images of the flip chip after the underfill curing process.

Figure 1. Top view images of the flip chip after the underfill curing process.

Failure to perform the plasma treatment will cause the underfill to bleed out, and imbalance on the dispensing side becomes relatively larger when compared to the opposite side. This difference is noticeably reduced in the plasma-treated flip chip. Therefore, it is apparent that plasma treatment aids in improving the uniformity of the underfill along the edge of the flip chip. Figure 2 shows the underfill performance from the side view.

Side view images of the flip chip after the underfill curing process.

Figure 2. Side view images of the flip chip after the underfill curing process.

The fillet height difference between plasma-treated and untreated samples can be observed at the corners of the device. In comparison to the untreated flip chips at the corners, the underfill material occupies an additional area in the case of the plasma-treated flip chips. This indicates that the surface energy is improved by plasma treatment, leading to better fillet height along the flip chip edge and enabling underfill fluid to surround the corners of the chip at the time of the underfill dispensing process.

Figures 3 and 4 show the optical images of CSP packages after underfill curing. This is the same as shown in Figure 1 and Figure 2. The plasma treatment prior to underfill improves the underfill performance by increasing wicking speed, balancing fillet height, and improving fillet uniformity.

Top view images of CSP after the underfill curing process.

Figure 3. Top view images of CSP after the underfill curing process.

Side view images of CSP after the underfill curing process.

Figure 4. Side view images of CSP after the underfill curing process.

Plasma for Adhesion

Poor adhesion between the metal finishes and the underfill, between the die passivation layer and the underfill, and between underfill and substrate will cause the interface to delaminate in flip chip packages. Delamination of a flip chip package is usually seen at the interface between the underfill material and the die passivation layer around the edges of the die. Many factors have an impact on underfill adhesion, like surface contamination and chemical nature of the passivation layer.

Plasma has the ability to remove impurity and can chemically change the surface for better adhesion. Generally, the temperature cycling test is used for assessing interface delamination, and confocal scanning acoustic microscopy (C-SAM) is used for evaluating whether the devices undergo the temperature cycling test.

Table 2 shows the C-SAM analysis results after the temperature cycling test. Gold-finished flip chips were the samples used for this assessment.

Table 2. Result of temperature cycling test

Substrate Finish   No. of Cycles
1000 2000 3000 4000
PCB AU Plasma 0/58 0/58 0/58 0/58
No Plasma 0/62 0/62 1/62 6/62
PCB AU Plasma 0/50 0/50 0/50 0/50
No Plasma 0/40 0/40 1/40 2/40

 

The temperature cycling range is −40 °C to 125 °C. Based on the results, the two untreated samples displayed a failure rate of 11.3% and 7.5%, respectively, whereas the O2 plasma-treated samples exhibited no device failure even after 4000 temperature cycles.

T-peel and Lap Shear tests were used to quantify the adhesion force, so that the plasma performance for enhanced adhesion can be established (see Figure 5).

Shear and peel force comparison between the plasma-treated and untreated samples.

Figure 5. Shear and peel force comparison between the plasma-treated and untreated samples.

A FR-4 coated with a solder mask 125 µm-thick polyimide film and a Henkel underfill material were utilized for testing. While the sample thickness was 10 mm, the adhesion area was 10 x 25 mm.

Two plasma modes were used for this assessment. IFP plasma was operated in the XTRAK-IFP system using 45 sccm of oxygen gas, 200 mT pressure for 120 seconds, and 200 W RF power. Direct plasma was operated in an AP-1000 plasma treatment system using 120 sccm of argon or oxygen gas, 200 mT pressure for 120 seconds, and 400 W RF power.

As illustrated in Figure 5, the strength of the interface adhesion improves after the plasma treatment, boosting the shear and peeling forces by as much as 216% and 435%, respectively. Moreover, at the time of the shear test, the failure mode is changed to polyimide film cohesion failure from polyimide-underfill adhesion failure. These outcomes indicate that a plasma-treated surface can improve the adhesion and decrease or remove the delamination in flip chip packages.

Conclusion

As demonstrated in this article, plasma treatment can be successfully utilized in capillary underfill processes. This kind of treatment significantly improves underfill dispensing adhesion and also enhances the performance at the interface, thereby decreasing or eliminating the delamination at the interface. Moreover, visual analysis demonstrated the enhancement in the uniformity and height of the fillet after the plasma treatment. Oxygen plasma treatment also reduces or eliminates device failure, and therefore, the reliability of devices is increased.

This information has been sourced, reviewed and adapted from materials provided by Nordson MARCH.

For more information on this source, please visit Nordson MARCH.

Citations

Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    Nordson MARCH. (2019, September 26). Plasma Treatment in Flip Chip Packaging. AZoNano. Retrieved on November 20, 2019 from https://www.azonano.com/article.aspx?ArticleID=5204.

  • MLA

    Nordson MARCH. "Plasma Treatment in Flip Chip Packaging". AZoNano. 20 November 2019. <https://www.azonano.com/article.aspx?ArticleID=5204>.

  • Chicago

    Nordson MARCH. "Plasma Treatment in Flip Chip Packaging". AZoNano. https://www.azonano.com/article.aspx?ArticleID=5204. (accessed November 20, 2019).

  • Harvard

    Nordson MARCH. 2019. Plasma Treatment in Flip Chip Packaging. AZoNano, viewed 20 November 2019, https://www.azonano.com/article.aspx?ArticleID=5204.

Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit