Process Benefits of Underfill Encapsulants for CSPs and BGAs
Underfill encapsulants were originally developed to encapsulate flip chip ICs. A silicon flip chip has a much lower coefficient of thermal expansion (CTE) than the substrate it is assembled on.
By Karl Loh and Edward Ibe
In thermal cycling, there is relative movement of the flip chip and the board, resulting in mechanical fatigue and failure of solder joints that electrically interconnect the chip to the board. An underfill encapsulant, typically an epoxy composition, wicks under a flip chip by capillary action, then is cured. It provides mechanical reinforcement to the solder joints, increasing life of the flip chip.
Use of underfill encapsulants has proliferated to encapsulation of surface mounted components, such as CSPs and BGAs. These components normally are capable of surviving thermal cycling requirements without being encapsulated, but are not designed to withstand repeated mechanical shock. For example, CSPs and BGAs are used in mobile phones, which are often dropped, and in automotive and military electronics, which must survive years of vibration and/or severe shock.
For CSP and BGA underfill encapsulation, the proper encapsulant is easy to handle and process, providing the required reliability. Parameters such as storage conditions, pot life, dispensability, underfill flow speed and cure time are paramount to handling and processing. If board value is high, a reworkable underfill might be used. In such cases, ease of rework is important. Void-free encapsulation, drop test life and thermal cycle life are important issues in reliability of handheld devices. For mobile phones, since the printed circuit board (PCB) is directly under the keypad, keypad actuation is important. For automotive and military electronics, vibration and more severe thermal cycling become critical. This article focuses on processability.
Storage Conditions
Traditional flip chip underfill encapsulants must be stored at -40°C. This often is inconvenient for the typical surface mount operation, where the coldest storage facility typically is a refrigerator operating at 5°C for solder paste storage. BGA and CSP underfill encapsulants can be stored at -5°C with a six-month shelf life. Acquisition of a freezer is required, but -5° C can be achieved even with a low-cost residential freezer. Work is being done to validate extended shelf life in a 5°C refrigerator.
Pot Life
Pot life is the useful life of an underfill encapsulant after it is removed from the freezer and thawed, typically determined by the ability to dispense uniform quantities at uniform speeds. Thus, viscosity must be stable over the length of the pot life. One definition of end of pot life is the time for viscosity to rise 10 percent.
In a typical underfill operation, encapsulant is dispensed onto boards preheated to as high as 90°C in a dispensing chamber that can be as hot as 40°C. At 40°C, an encapsulant with a two-day pot life at room temperature may only have an eight-hour pot life. The shorter the pot life, the smaller the package size needs to be for the contents to be consumed within its usable life. In high-volume operation, this results in frequent shutdowns of the dispenser to change a syringe. With a short pot life material dispensed in a hot chamber, there also is a higher risk of material setting up in the line or in the dispensing needle, resulting in the need for time-consuming cleanup. With a longer pot life material, large cartridges can be used.
Dispensability
Ease of material dispense affects both productivity and capital cost. Lower-viscosity, lower-density material can be dispensed through a small orifice needle faster and with less pressure than a material with higher viscosity and density. Traditional flip chip underfill encapsulants might have a viscosity of 15,000 cps or substantially higher. Because they are highly loaded with silica filler, specific gravities can be as high as 1.8. Dispensing such a material at high speed and precision requires positive displacement equipment, such as auger pumps or piston pumps.
Figure 1. Comparison of flow rate — flip chip underfill encapsulant vs. CSP underfill encapsulant
Many CSPs and all BGAs are substantially larger than flip chips, requiring higher flow rate to achieve comparable throughput. The latest underfill encapsulants designed specifically for CSP and BGA encapsulation have viscosities of 7,000 cps or lower. And because they are not even filled, some have specific gravities as low as 1.1. If shot-to-shot repeatability is not critical, these materials can be dispensed with a simple pneumatic system. To more precisely control shot size, the pneumatic system can be fitted with an economical spool valve. In a spool valve, a rod moves up and down to open and close an orifice.
Underfill Flow Rate
A CSP/BGA underfill encapsulant has lower viscosity and density than a typical flip chip encapsulant. This allows a CSP and BGA encapsulant to flow much faster, desirable since the volumes to be filled are much larger for a CSP or a BGA. One company*** standardizes the measure of flow rate by measuring the time to flow across overlapping glass slides. Two glass slides are overlapped by an 18-mm distance, separated by a 75-µm gap using shims. The sandwich is placed on a hot plate to heat it to the desired temperature, then encapsulant is dispensed and the time to traverse the 18 mm distance is measured.
Figures 2a and b. Effect of board moisture. Wet boards create voids during underfill cure
In Figure 1, the flow rate of a flip chip underfill encapsulant is compared with that of a CSP/BGA encapsulant, showing that lower viscosity and specific gravity indeed improve flow rate. In this comparison, at 90°C it takes 50 percent less time to traverse an 18-mm distance. This improved flow rate is critical to maintaining high line speed and productivity.
Void-free Encapsulation
Factors that affect the presence of voids include moisture in the board or component, flow profile of the encapsulant, and flux compatibility. Moisture trapped in a board or component can evolve during underfill curing, typically performed well above 100°C. Figure 2 compares the encapsulation of a BGA whose board has not been dried and the encapsulation of a BGA whose board has been pre-baked at 125°C for four hours. Many voids are seen with the unbaked board (a), while none are seen with the pre-baked one (b).
Figure 3. Underfill flow fronts. Flat flow front reduces risk of voids.
Flow profile also affects the possibility of a void appearing. In Figure 3, the flow profiles of two underfill encapsulants are compared. Encapsulant flows from right to left, with the leading edge of flow visible. In the image on top, the flow front is flat, conducive to void-free encapsulation. In the image on the bottom, the flow front is wavy. Fingers can close on each other to form voids.
Figures 4a and b. Underfill/flux compatibility. Good compatibility yields void-free encapsulation.
In Figure 4, components are attached with two different no-clean solder pastes. In one case, voids can be seen adjacent to the solder bumps. In the other case, the encapsulation is completely void-free. Although voids are rather small, those that bridge two adjacent solder bumps can be catastrophic. If the component is subjected to a subsequent reflow operation, the solder can flow and traverse the bridge, resulting in a short circuit.
Curing Time
Newer CSP/BGA encapsulants can be cured in an inline oven or a converted solder reflow oven in five minutes or less.
Curing of an underfill encapsulant can be measured by the use of a dynamic scanning calorimeter (DSC). A DSC measures the amount of heat released. When heat is no longer released, the reaction has stopped. A follow-on DSC analysis at higher temperatures will confirm whether or not the material is fully cured. Additional curing taking place at higher temperatures would indicate that the original cure schedule was inadequate.
Gradual temperature ramps are recommended, as rapid ramping of temperature to cure temperature and rapid cooling can cause excessive stress on an assembly.
Reworkability
Unlike flip chip assemblies, which often are single-chip packages, CSPs and BGAs are most often assembled on multi-component assemblies that can be of high value. If an underfilled component is found to be faulty, rework is required. With a conventional flip chip underfill, attempts to remove the component will damage the board and/or the solder pads. For this reason, reworkable underfills have been developed.
One development is a reworkable underfill that becomes soft and loses adhesion at 200° to 220°C. When heated to that temperature, which is also above solder reflow temperature, the component is easily removed and underfill residues are scraped off without damage to the soldermask and solder pads. Then, using conventional rework tools (soldering iron, solder wick and hot air gun) the solder pads are cleaned and prepared for attachment of a new component.
The table compares properties of a traditional flip chip underfill with those of two CSP/BGA ones, one a non-reworkable underfill and the other a reworkable one. Regarding uncured properties, no filler in the CSP/BGA underfills results in lower viscosities and specific gravities, resulting in easier dispensing and faster underfill flow. They also have longer pot lives at room and elevated temperatures.
With respect to the cured properties of the non-reworkable CSP/BGA underfill, glass transition temperature need not be sacrificed compared to the flip chip underfill; however, there is a substantial increase in the CTE. Commercial flip chip underfills have CTEs ranging from 20 to 40 ppm/°C. At 60 ppm/°C, CTE far exceeds what is typically desirable in a flip chip application.
Regarding cured properties of the reworkable underfill that is commercially used in the manufacture of mobile phones and military electronics, the material has a lower glass transition temperature as well as a higher CTE. To attenuate these properties for applications with more severe thermal cycle requirements, a silica-filled version is available with reduced CTE.
Conclusion
An underfill designed specifically for CSP/BGA encapsulation can provide significant benefits in processability and productivity. The absence of silica broadens the formulating latitude, allowing the manufacturer to offer a material that is easy to store, with long pot life, yet one that also cures rapidly in an inline oven. The absence of filler also reduces viscosity and specific gravity, resulting in substantially improved dispensability and flow rate. A tradeoff is made in cured material properties, specifically a higher CTE. Reworkable underfills also are available that offer similar process benefits.
*X6-82-5LV
** X13563
*** Zymet Inc.
Karl Loh and Edward Ibe may be contacted at Zymet Inc., East Hanover, NJ 07936, (973) 428-5245; E-mail:kloh@zymet.com and edwardibe@zymet.com.
SMT July, 2004
Author(s) : Edward Ibe Karl Loh
