Methods for gaining reliability in underfill applications
BY ALAN LEWIS
Voids or air gaps in underfill are a common problem across underfill applications, from the smallest die on flex to the largest BGA. The consequences of having voids in underfilled parts depend on the package design and use model. Voids typically result in a loss of reliability. This article explores strategies for troubleshooting void problems.
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Detecting Voids
If you have determined there is a voiding problem, you probably already have a method of detecting the voids; however, different methods can be useful for troubleshooting. Three of the most common methods for detecting voids are the use of a glass die substrate, ultrasonic imaging, and destructive testing of a cross section or breaking the die off the part.
Using a glass die or substrate can be helpful. This method provides instant feedback during testing and can be used to help understand flow patterns to optimize underfill speed. Using underfill materials of different colors can also help visualize the flow. The disadvantage of this method is that flow and voiding behavior may be slightly different for glass parts than actual production parts.
Ultrasonic acoustic imaging is a powerful tool. It allows the user to detect voids in the underfill material on the actual production part before or after cure. The size of the void to be detected can be limited depending on the package and equipment used, so there is a need to check with the equipment makers to understand what size void can be detected. These tools are also useful in reliability testing to detect delaminating and interconnect failures. Figure 1 shows an image of a void in an underfilled package taken with an acoustic microscope.
 Figure 1. Image taken with acoustic tomograph shows void in underfilled package. Photo courtesy of Hitachi Kenki FineTech. |
Destructive testing uses a cross-section saw or breaks the die or package away from the underfill. These methods can be useful to better understand the three-dimensional shape and position of the void. The primary disadvantage of this method is that it cannot be used on uncured parts.
Causes of Voids
There are several potential root causes of voids. Describing them and their root causes helps devise tests to troubleshoot them. Some causes include:
- Flow pattern voids. There are several sub-categories here, but all of these voids occur during the time the material is flowing under the die or package. The leading edge of the wave front traps a pocket of air.
- Moisture voids. This type of void occurs during curing when moisture from the substrate outgases. This commonly occurs in organic substrates.
- Voids caused by bubbles in the fluid. This is rare in fluids that materials suppliers packaged, as most suppliers are careful about packaging materials air-free. However, mishandling the fluid or repackaging after receipt from the suppler can introduce bubbles. In some cases, suppliers provide samples or experimental fluids that may not be properly de-gassed. If not configured properly, some automated dispensing equipment can also induce bubbles in the fluid path during dispense.
- Contamination voids. Contamination of excess flux or other sources of contamination can occur in a variety of ways.
Void Characteristics
Void characteristics can help match them up with their root causes. These include:
- Shape – Are the voids round or some other shape?
- Size – Voids are usually described as the area they cover in the plane of the die.
- Frequency – Do you get about one void per 10 parts, or 10 voids per part? Do voids occur during specific times, all the time, or randomly?
- Location – Do the voids appear in one place of the die or randomly? Do they appear attached to interconnect bumps? What is the relationship of the void to the dispense pattern?
Test Strategies
The first step is to determine if the voids occur before or after curing. This can be helpful in eliminating some root causes. If the voids are not present after dispensing, but are present after curing, flow pattern voids, or voids caused by bubbles in the fluid, can be eliminated as a root cause. At this point, it would be good to look for moisture problems, contamination problems, some source of outgassing during cure, or problems with cure profiles. Most underfill materials are designed to shrink during cure to create compressive stress on the interconnect bumps to improve reliability. This shrinking can give any outgassing source the ability to create a void. If the voids are present with the same characteristics before and after cure, it is a good indication that some flow pattern during the underfill process caused the void. If the number of voids changes after cure, there could be more than one root cause. In some cases, contamination can cause two different types of voids; they can create an obstruction during flow, then outgas during cure.
Flow-pattern Voids
Two or more flow fronts meeting to trap a pocket of air cause flow-pattern voids. One cause of this can be the dispense pattern. Dispensing on multiple sides of a BGA or die can improve the speed of the flow, but increases the probability of trapping a void. Experimentation with various dispense patterns or parts with a quartz die or transparent substrate is the most direct method of understanding how the voids are formed and how to eliminate them. The use of underfill materials with different die colors for various dispense passes (Figure 2) can be a good tool to visualize flow.
 Figure 2. Use of glass and two colors of underfill can help visualize flow and formation of flow-pattern voids. |
Temperature can affect the flow front of the material. Temperature variations on the part can also affect material cross-linking during flow, speed of flow, and flow speed. Therefore, it is prudent to consider this variable in testing.
Often, multiple dispense passes are used to reduce fillet size, but can also increase the probability of trapping voids if timing between the passes is not carefully planned and controlled. The use of jetting technology, instead of needle dispensing, to control fillet size can help reduce the number of passes.1 Figure 3 shows jet dispensing of underfill.
 Figure 3. Jet dispensing the underfill, rather than needle dispensing, can avoid some causes of voiding under the die. |
Material flowing to other board features (passive components or vias), leaving the underfill material short, can also cause flow-pattern voids. The use of jetting technology can help control the placement of the underfill fluid.
Moisture Voids
Moisture in the substrate can outgas during cure, creating a void during the cure process. These voids are often random in placement and can have finger- or snake-like shapes. They usually are seen in packages using organic substrates.
To test if voids are caused by moisture, one can pre-bake the parts for several hours at temperatures above 100°C, then dispense immediately on the parts. Once it has been established that moisture is the root cause, further testing to establish optimal pre-bake times, temperatures, and storage protocols can be designed. A good metric for water content is to track weight gain of a part with a precision analytic balance.
Note that some flux contamination issues can be remedied with a pre-bake procedure and act like moisture-induced problems. It is easy to test for the difference. Moisture-induced problems will recur if the part is exposed to humidity; flux contamination problems will not.
Contamination issues caused by excess flux often create irregular or random flow variations, particularly at the interconnect bumps. If the voids that are occurring during flow show this characteristic, it would be prudent to investigate cleaning or sources of contamination. In some cases, flux contamination can show up after cure in a series of small bubbles on the side of the die opposite the dispense side. Apparently, fluid flow carries the flux to the far side of the die.
Material Bubble Voids
As mentioned earlier, most material suppliers are very careful about packaging underfill material with air bubbles. Improper handling, repackaging, or dispensing technology can induce these issues. If air bubbles in the material are suspected, there is a straightforward way to inspect for this. Dispense the material from the syringe through a fine needle and draw a fine line in a long pattern, then inspect for gaps in the dispensed line. If bubbles in the material have been confirmed, contact your material supplier about proper handling and storage of the fluid.
If no bubbles are found, this test can be repeated with the valve, pump, or jet attached to the syringe. If voids occur during this test, and no voids were present when dispensing directly from the syringe, then the equipment induced the bubbles. In this case, contact your equipment supplier about proper setup and equipment use.
Conclusion
Underfill voids can be a vexing production problem. Understanding the characteristics of various root causes, and how to test for them, can help engineers resolve the issues.
References
- Babiarz, Alec J., Paradigm Shift in Applying Underfill, Pan Pacific Microelectronics Symposium, SMTA, 2005.
ALAN LEWIS, director of application engineering, may be contacted at Asymtek, 2762 Loker Ave. West, Carlsbad, CA 92010; 760/930-3379; e-mail: [email protected].