Anna Lifton, Westin Bent, Frank Andres, Irina Lazovskaya and Jason Fullerton
Alpha Assembly Solutions,
Bottom Terminated Component (BTC) use has become extensive in electronic assembly, especially the use of Quad Flat No-lead (QFN) packages. Low cost and small size with improved thermal and electrical performance make BTC components very attractive for many applications.
Implementation of BTC components come with challenges. The low standoff, large central thermal pad and lack of leads result in co-planarity issues due to board warpage and Coefficient of Thermal Expansion (CTE) mismatch. Low standoff and a large central pad may favor increased voiding compared to other package designs. The differences in shape, size, and orientation between the large central/thermal pad and small perimeter pads create different demands on the solder paste, making paste deposit volume and shape consistency across the package difficult to achieve. The fine feature apertures are susceptible to skips and insufficient volume. The large open apertures are susceptible to scooping and drag out. This varying demand on the solder paste rheology complicates paste selection requirements.
In this study, two solder paste chemistries and several printing parameters were evaluated in order to achieve consistent solder paste deposition across the assembly. There are tighter tolerance requirement on the solder paste volume due to thinner stencils and the need for consistent solder paste uniformity across the footprint of the part to prevent potential tilting of the lower standoff components. Printing parameters can be adjusted to ensure well defined (no scooping/drag out), defect free deposits on the thermal pad. New solder paste chemistries with improved rheological properties have been formulated to print with lower squeegee blade angle and pressure which mitigates the scooping phenomena typically observed with larger aperture. Lowering squeegee blade angle with these formulations also appears to improve printing quality for all aperture sizes without the increased flux leaching normally associated with lower print angle. Reduction in print pressure and contact angle will reduce wipe frequency and wear on the stencil. Improved formulations result in lower voiding levels and more uniform print deposition across large and small apertures.
A new generation of solder paste has been developed to specifically target BTC assembly challenges in particular voiding and solder print consistency This study investigated the effect of paste rheology, print blade angle, and print process settings on the height consistency of print deposits for large and small features.
The most important finding is that older formulations of solder paste may not be effective at printing the large center terminations and perimeter signal terminations found on many BTC devices, with respect to the deviation in height of large center features and small signal features. The new generation solder paste tested in this study, with improved rheology, was capable of printing with good volume performance and good height consistency under a variety of process settings.
The most significant process setting was the use of a 45° blade angle. This enabled a very large window of print settings that provide low height deviations across large and small features simultaneously when used with the new generation solder paste. This also enabled height consistency with the old generation paste under a smaller set of conditions, where a 60° blade angle demonstrated an inability to meet the same level of height consistency.
The new generation solder paste was not only capable of meeting the printing challenges posed by having QFN components on an assembly along with a mix of other fine feature devices, but it also gave very good thermal pad voiding results as well.
Initially Published in the SMTA Proceedings