Reliability Comparison of Leaded, Array, & Leadless Packages on Alternative PWB Finishes

Increasing customer demands to provide more functionality in smaller electronic devices has consequently driven the urgency to increase I/O density and reduce the size of integrated circuits (IC). This has led to the widespread use of ball grid array (BGA) packages and fine pitch array packages, most of which have pitches ranging from 1.27 mm down to 0.5 mm. More recently, leadless packages such as the Quad Flat Pack No-Lead (QFN) and micro leadframe package (MLP) have provided electronics designers even more options to increase circuit density. These packages are characterized by having lands on the bottom surface periphery of the package (2 or 4 sides), which are soldered to the PWB using only the volume of solder deposited at the screen printing process. In addition, they have a centered thermal pad soldered to the PWB, which serves to draw heat away from the die and provide greater reliability. The advantages of using leadless packages over the more conventional leaded QFP, PLCC, and SOIC include reduced thermal impedance, improved electrical performance, lower profile, and reduced real estate required (more than 50% reduction in many cases). Further, since the cost of producing these types of packages is lower for the IC assembler, the purchase price for the end user makes them an attractive alternative to their leaded counterparts. One of the more immediate concerns with using these leadless packages is one of reliability. Unlike array packages which have a sphere of solder in addition to the paste print solder volume, the QFN and MLP use only the paste print. Consequently, the stand-off height after reflow soldering is approximately 2-3 mils. This means that the materials coefficient of thermal expansion (CTE) values must be carefully considered. Another related concern is the need for a flat surface on the PWB. Due to the inherent thickness variability of hot air solder leveled (HASL) boards, it would be expected that this would not be appropriate for use with fine pitch array or leadless packages. Alternative finishes such as immersion gold over nickel (ENIG), organic solderability preservative (OSP), flash gold over nickel, immersion silver (ImAg), immersion tin, etc., are all being used in the electronics industry to provide a consistently flatter surface for placing these types of devices.