Novel Approaches to Current BGA Packaging Challenges

Despite the tackiness offered by ball attach fluxes, they have failed to prevent ball movement on solder on pad finishes when the solder protrudes above the solder mask. Solder bridging and joined balls become a common occurrence prompting heavy rework of units. Rework lengthens the manufacturing cycle time and poses a product reliability risk. The same ball movement plagues ENIG pads on FC ceramic substrates. These solder ball pads do not provide a solder mask edge to contain the solder balls, as there is no organic solder mask coating. Dippable solder paste provides the tackiness needed for these adverse situations as well as small solder particles that inhibit ball rolling due to mechanical or convective forces downstream. Dippable solder paste also has excellent rheological properties that allow it to be used in pin transfer yielding consistent deposits of paste onto the ball pads without smearing adjacent pins unlike flux. Flux in pin transfer application will need to be wiped off from the pins after a few transfer intervals to prevent flux bridging between pins which would most likely lead to solder bridging or joined balls during reflow. This results in inevitable equipment downtime as this process is manual. It has been shown that dippable paste overcame these process challenges. Today, the most popular ball attach pad finish on high performance flip chip BGA's with high I/O's is solder on pad. Immersion Sn comes in second with oxidation an issue. By far, the OSP pad finish gives the cheapest substrate cost. However, partial wetting has been observed with the flux only ball attach process because of uneven OSP coating, too thin of a coating results in the quick oxidation of pads, too thick of a coating prevents total solder wetting. The use of dippable paste has also demonstrated relief in these OSP related issues. In this paper, it has been shown that dippable paste shows no partial wetting on OSP pad finish. This can be attributed to the robust flux chemistry and the effect of powder on wetting during reflow. The challenge with finer BGA ball attach pitch is flux bridging between flux transfer pins. The key to this is dippable paste. Dippable paste has a rheological property that is very consistent and does not allow for bridging between pins. The future for BGA is finer pitches and dippable paste makes that possible for electronic packaging assembly lines. Pin transfer simulation has facilitated the development of dippable paste formulations. A description of typical transferred deposit attributes is discussed. Lastly this paper includes an overview of stacked BGA's also known as Package on Package no clean dippable solder paste development. Unlike the water wash formulations common and critical to the ball attach process, these are high reliability no clean dippable formulations. Although tacky flux has been successful in many PoP applications, issues such as coplanarity and package tolerance that require increasing the sphere size during assembly, have found benefit from dippable solder paste.