Whitepapers

Thermal Pad Design and Process for Voiding Control at QFN Assembly

Dr. Ning-Cheng Lee, Dr. Yan Liu

Summary: Quad-flat no-leads (QFN) package designs are receiving more and more attention in the electronics industry. This package offers a number of benefits including (1) small size, such as a near die size footprint, thin profile, and light weight; (2) easy PCB trace routing due to the use of perimeter I/O pads; (3) reduced lead inductance; and (4) good thermal and electrical performance due to the adoption of exposed copper die-pad technology. These features make the QFN an ideal choice for many new applications where size, weight, electrical, and thermal properties are important. However, the adoption of QFN often runs into voiding issue at SMT assembly. Upon reflow, outgassing of solder paste flux at the large thermal pad has difficulty escaping and inevitably results in voiding. It is well known that the presence of voids will affect the mechanical properties of joints and deteriorate the strength, ductility, creep, and fatigue life. In addition, voids could also produce spot overheating, lessening the reliability of the joints. This is particularly a concern for QFN where the primary function of thermal pads is for heat dissipation. Thermal pad voiding control at QFN assembly is a major challenge due to the large coverage area, large number of thermal via, and low standoff. Both design and process were studied for minimizing and controlling the voiding. Eliminating the thermal via by plugging is most effective in reducing the voiding. For unplugged via situations, a full thermal pad is desired for a low number of via. For a large number of via, a divided thermal pad is preferred due to better venting capability. Placement of a thermal via at the perimeter prevents voiding caused by the via. A wider venting channel has a negligible effect on voiding and reduces joint continuity. For a divided thermal pad, the SMD system is more favorable than the NSMD system, with the latter suffering more voiding due to a thinner solder joint and possibly board outgassing. Performance of a divided thermal pad is dictated by venting accessibility, not by the shape. Voiding reduction increases with increasing venting accessibility, although the introduction of a channel area compromises the continuity of the solder joint. Reduced solder paste volume causes more voiding. Short profiles and long hot profiles are most promising in reducing the voiding. Voiding behavior of a QFN is similar to typical SMT voiding and increases with pad oxidation and further reflow.