by Joe Cannella
It may not be a sound economic decision to reball and reuse a BGA component that has a low initial cost. However, many custom ASIC's and even some "off the shelf" BGA components, because of their complexity, can be very expensive with limited availability and excessively long lead times. For these types of components it can be cost effective to have the ability to reuse components that have failed due to soldering defects. In this article we will discuss some considerations that must be addressed in order to successfully perform reballing of BGA components.
It is important to know the component package specifications before attempting a reballing process. These specifications include maximum thermal limits for the component materials, alloy type (either eutectic or high temperature), ball size, moisture sensitivity level and most importantly the manufacturer's recommendations for the maximum number of reflow cycles the component can withstand. This information can be obtained from the component data sheets or directly from the component manufacturer.
Component Substrate Preparation:
Before attaching the new component interconnects the component substrate must be carefully prepped by removing all residual solder. The most efficient method is to use solder braid and a wide blade soldering iron tip. The use of flux during this process will increase the effectiveness of the solder braid in wicking the residual solder from the component substrate. Because of the increased risk of component substrate damage, care must be taken to avoid “scrubbing” the substrate surface with the solder braid and iron tip during this process.
Once all of the residual solder has been removed from the component substrate, it should be thoroughly cleaned using isopropanol alcohol (IPA) to remove any remaining flux residues. The component substrate should then be inspected for any evidence of component substrate damage.
Reballing Process Options For Eutectic Interconnects:
Once the component has been properly prepared for attachment of the new interconnects (balls), a process for accomplishing this task must be selected. The three main options available (see figures 1, 2 and 3) when reballing with eutectic balls are either the preform method, the screen method or a relatively new method based on the screen method which employs a vacuum pick-up for holding the alloy spheres in place. The screen method requires specialized fixturing to place individual solder balls over the corresponding component substrate land pattern. Once all of the lands have a new ball in place, the entire fixture is sent through the reflow process to melt the solder balls onto the component. An alternate method is to use solder preforms in conjunction with a simple frame sized to match the outside dimensions of the component. This method has proven more efficient and reliable than the screen method. The solder ball preforms are available in literally thousands of package configurations and are very easy to use. The preform consists of precisely spaced balls sandwiched between a lamination of cardboard that has been impregnated with a water-soluble flux. Simply apply a water-soluble tacky flux to the component substrate, place the preform onto the component and then place the component into the frame. Reflow the component in the frame to melt the solder balls onto the component substrate. Once the solder preform balls have wetted to the component substrate and resolidified the cardboard can simply be peeled away from the component and cleaned using DI water. This method, although more efficient, does require a bake out cycle of the reballed components for approximately 24 hours prior to placement of the components onto an assembly.
Reballing Options For High Temperature Interconnects:
When attaching high temperature interconnects to a component, the only viable option is to first screen solder paste onto the component substrate using a solder paste stencil and then placing the high temperature preform into the screened solder paste. The use of a split vision rework station helps in alignment of the preform to the component. The thermal profile requirements for reflow are also more critical when working with high temperature interconnects as opposed to eutectic. It is critical that the thermal profile be of sufficient heat to fully reflow the eutectic solder paste without reflowing the high temperature interconnect and remain within the maximum component temperature rating.
Cleaning per flux chemistry:
The cleaning process of the reballed component will depend upon the type of flux chemistry used during the reballing process. If using a no-clean or rosin based flux chemistry, isopropanol alcohol will do a very good job of removing flux residue. When using the solder preform method, which requires the use of water-soluble flux, cleaning in DI water is necessary. Because of possible water absorption by the component during the DI water cleaning process, it is necessary to preform an additional process step of a bake-out cycle of the component prior to use. The recommended bake-out for standard PBGA packages is 24 hours at 125 degrees Celsius. Other component configurations may have different recommendations, which can be obtained from the component data sheet.
When properly performed, the reuse of BGA components is possible and can be very cost effective. Understanding and adherence to the component and process specifications is critical to success. With experience, reballing and reuse of BGA components can be achieved rather easily. The EMPF offers a two-day curriculum on BGA processing and rework which covers all aspects of BGA reballing methods. If you would like additional information please contact the EMPF helpline at 610-362-1320 or log onto the EMPF website at empf.org.