Abstract
Overmolded Flip Chip Quad Flat No-lead (FCQFN) is a low cost flip chip on leadframe package where there is no need for underfill, and is compatible with Pb free or high Pb metallurgy. A robust leadframe design, quality solder joint formation and an excellent molding process are three factors needed to assemble a high performance FCQFN. It combines the best of both wirebonded QFN and wafer chip scale devices. For example, wafer chip scale has low resistance, but inadequate thermal performance (due to absence of thermal pad), whereas wirebonded QFN has good thermal performance (i.e., heat dissipated through conductive die attach material, through the pad and to the board) but higher resistance. Flip chip QFN combines both positive aspects – that is: low resistance and good thermals.
One of the common defects for molded packages across the semiconductor industry is the occurrence of mold voiding as this can potentially affect the performance of a device. This paper will discuss how mold voiding is mitigated by understanding the mold compound behavior on flip chip QFN packages. Taking for example the turbulent mold flow observed on flip chip QFN causing mold voids. Mold compound material itself has a great contribution to mold voids, hence defining the correct attributes of the mold compound is critical. Altering the mold compound property to decrease the mold compound rheology is a key factor. This dynamic interaction between mold compound and flip chip QFN package configuration is the basis for a series of design of experiments using a full factorial matrix. Key investigation points are establishing balance in mold compound chemistry allowing flow between bump pitch, as well as the mold compound rheology, where gelation time has to be properly computed to allow flow across the leadframe.
Understanding the flow-ability of mold compound for FCQFN, the speed of flow was optimized to check on its impact on mold voids. Mold airflow optimization is also needed to help fill in tighter bump spacing but vacuum-on time needs to be optimized as well.
I. Introduction
With the advancement of technology, new device packages are being developed and created at a fast rate. This advancement in technology brings the need to provide good power management products. The semiconductor industry continues to demand increased densities of device integration coupled with diminished component feature sizes which enables increased power and thermal energy generation. Wirebonded QFN and WCSP make a good candidate to provide increased power density because of the direct current flow on interconnects. However, both package configurations are bad on thermals due to limited direct thermal path from die to the board. Flip Chip QFN has been a viable package for small outline devices with advantages over other plastic body packages making it an ideal package type for thermal energy dissipation. The combination of flip chip interconnects and QFN package is now a solution that merges the need of small package with high power density and good thermal dissipation. In addition, flip chip QFN is the closest chip-scale QFN which is flexible and easy to assemble.
A. Flipchip QFN
Flip chip QFN is a flip chip interconnect in a QFN body as shown in Figure 1. Combining flip chip interconnect with a QFN body brings about the advantages of good thermal efficiency for the QFN body and the low resistances giving high electrical efficiency advantage of a flip chip interconnect. With the elimination of wirebond process it also brings the advantage of near chip scale QFN having a very small die to mold gap. Flip chip QFN now gives a device with high power density and good thermal efficiency. Another good advantage with flip chip QFN technology is its shorter cycle time over wirebonded devices since wirebonding log point will be removed.
B. Flipchip QFN Device Complexity
The growing interest on flip chip QFN package also brings about new devices with more advanced functions and features catering to wider range of industry needs. These new features and functions equates to a more complex product designs. The unique features of complex flip chip QFN devices can pose new challenges during product assembly. One of the potential challenges would be on mold voiding for fine bump pitch, where understanding the mechanism how mold voiding develops is essential to eliminate its occurrence on flip chip QFN package.
C. Mold Voiding Mechanism
Mold voiding is an occurrence of which air is entrapped inside a package causing voids during mold process. The unique package construction of flip chip QFN devices provides added difficulty in easily detecting mold voiding as conventional mold void detection methods do not work. It is critical then to establish a new and accurate detection methodology for mold voiding. To understand further mold voiding mechanism on flip chip QFN, a series of mold short shot mapping results at mold process were meticulously analyzed. It was observed that the mold compound tends to flow first on the unit periphery because of the resistance of flow on the interconnect area allowing for it to converge on the center of the unit causing entrapped air or mold voiding commonly on the interconnect area.
II. Experimental Section
A. Mold Voiding Mitigation Factors
Understanding the mold void mechanism helped the team to establish mitigation factors to eliminate mold voiding on flip chip QFN Devices.
1) Mold voiding detection methodology
To eliminate mold voiding it is critical to establish an effective and accurate detection methodology. Most common mold voiding detection method is using scanning acoustic microscopy (SAM). This is effective for units with very large mold voids but it is not effective for marginal or minute mold voids. To effectively mitigate mold voiding the team was able to innovate and create a new methodology to enable accurate detection of mold voiding. This concept focuses on in-house decapsulation exposing the bump area.
2) Mold Compound Material Storage Condition
Mold compound itself is a mixture of different components that contributes to its excellent properties. These components make the material very sensitive to temperature and moisture. Uncontrolled temperature exposure can change material properties of mold compound which can impact flow ability during molding process thus can result to mold voiding. The absorption of moisture can also contribute to mold voiding as the moisture can create vapors during mold process eventually leading to voiding.
3) Mold Flow Behavior
Mold flow behavior is one of the critical factors to consider in eliminating mold voiding. Understanding how the mold compound flow behaves across the strip helps in eliminating severe backflow and air entrapment that can potentially cause mold voiding.
4) Mold Formulation Modification and Chip to Leadframe Confluence
Formulating the proper mold compound compatible with the manufacturing process is a key component as it helps the overall strength of mold material. Certain mold compound additives and its specific geometric configuration can aid the molding process depending on property and function. Mold compound additives impact the behavior of material during molding thus it needs to be considered to ensure good mold flow ability in between gaps inside the flip chip QFN package components. One way to improve flow ability is understand the confluence between chip and leadframe clearance.
5) Mold Cavity Airflow
Presence of air inside mold chase during mold process can generate air resistance that allows some level of drag force. This causes mold flow to lag-off on the center area of panel during molding process. This phenomenon allows mold to flow first on corner area of panel and generate backflow causing air entrapment and eventual mold voiding. By optimizing air flow-ability to remove the presence of air inside the mold chase helps the flow of mold compound. This ensures a very even mold flow minimizing occurrence of backflow and entrapped air.
6) Mold Compound Rheology
Mold compound rheology is also critical for molding process as it defines the distance mold compound can flow before solid state. This property can impact mold voiding since it would dictate how mold material will perform for different device lay-outs as well as leadframe dimensions. It is critical to note mold compound rheology as these properties go hand in hand with defined mold parameters
III. Results and Discussion
A. Mold Voiding Mitigation Results
Performing DOE on all identified factors, the results for each individual factor and response is showed on Table I. Each factor showed improvements on the mold voiding occurrence, but results showed that no individual response or factor can totally eliminate mold voiding.
1) Mold Compound Material Storage Condition
Assessment on the impact of mold compound material condition shows that it affects mold compound flow ability. When exposed to uncontrolled temperature and open air, the mold compound property significantly changes. This is because the premature crosslinking of mold components thus having impact on mold flow ability resulting to voiding. The prolonged storage also of mold compound material will reduce overall mold compound rheology resulting to occurrence of mold voiding as will be discussed on the latter part of this paper.
2) Mold Flow Behavior
Optimizing how mold flow behaves on units across the strip showed mold voids improvement. As showed on table I, both factors 2.1 and 2.2 on mold flow behavior showed improved mold voiding but one factor has increased mold bleed-out. Mold bleed-out was mitigated by optimizing the mold mechanical set-up. Mold void response was revalidated with new mold set-up and yielded the same result.
3) Mold Formulation Modification and Chip to Leadframe Confluence
Table I shows impact of mold formulation modification and optimizing confluence between chip to leadframe. Mold voiding signature mapping shows that mold formulation modification and optimizing the confluence between chip and leadframe allows better movement of molding compound which gives a better response in eliminating mold voiding in between bumps.
4) Mold Cavity Airflow
Mold cavity airflow optimization significantly improved occurrence of mold voiding as shown in table I. Mapping out the mold voiding signature, it is observed that optimized airflow is effective in eliminating gross mold voiding. Optimized airflow improves mold flow stability considerably improving gross mold voiding but not able to address voiding in between bumps.
5) Mold Compound Rheology
Mold compound samples with different rheology were evaluated to check for response on mold voiding. It was validated that the increased mold compound rheology has better response on mold voiding as shown in figure 2. Consistent zero mold voids response was achieved for higher mold compound rheology value. Mold material storage impacts mold compound rheology overtime thus it is critical to consider degradation of mold rheology.
6) Combination of Factors to Eliminate Mold voiding
By understanding each contribution of different factors, the optimal combination of factors to eliminate mold voiding is derived. By applying these combinations of factors and ensuring the mold compound rheology limit, the team was able to consistently process package with zero mold voiding.
IV. Conclusion
Continuous advancement in flip chip QFN innovation and technology calls for new products with different flavors. Inevitably, as technology evolves challenges manifest in providing a robust package. Potential risks that come along with these complexities and ways of eliminating mold voiding were identified. To eliminate mold voiding on a flip chip QFN package, it is critical to understand the mechanism of how the mold material behaves during molding process resulting to voiding. It is also necessary to establish an accurate detection methodology and identify the best combination of factors. The best combination of factors to eliminate mold voiding as validated on this paper by going for
1.) Optimized mold flow behavior
2.) Modified mold formulation
3.) Understanding confluence between chip to leadframe
4.) Optimizing mold cavity airflow
Understanding mold compound properties such as ensuring high mold compound rheology limit and its impact on manufactured product is very critical to guarantee good and reliable products.
The identification of each factor to guarantee package robustness will help to ensure good products of flip chip QFN are released to market.
Acknowledgment
The authors would like to acknowledge and thank the following for their contributions in the completion of this paper:
TI Semiconductor Packaging Clark team,
TI Semiconductor Packaging Modelling Team
TI Semiconductor Packaging Dallas,
TI Clark Operations team,
TI Clark FA team,
TI Clark QRA team,
