Solder Pastes

The external interconnection features (such as leads, bumps, or balls) of surface mount devices (SMD's) are usually soldered onto a printed circuit board (PCB) through a board mounting process that consists of three basic steps: 1) application of solder paste on specific locations on the PCB, or solder paste printing; 2) positioning of the components on the board; and 3) solder reflow.

The solder paste, which serves primarily as the attachment medium between the device interconnection features and the PCB itself, is a specially blended paste that consists of a flux medium containing highly-graded solder alloy powder particles. The components of a solder paste are designed to give it excellent printing and reflow characteristics. The term 'reflow' refers to the process of exposing the solder paste to elevated temperatures (e.g., 230-260 deg C) to 'melt' the solder particles and allow the liquid solder to form a good and reliable connection with the board-mounted devices.

Aside from serving as the source of solder material that forms the mechanical and electrical connection between the SMD's and the board, the solder paste also accomplishes the following: 1) keep the components in place on the board prior to the reflow process; 2) clean the solder landing sites on the PCB as well as the external interconnections of the components; and 3) prevent these PCB solder lands and device interconnections from oxidizing until the soldering process is completed.

According to experts, there are several things to consider when choosing the right solder paste. These solder paste selection criteria include: 1) the size of the solder alloy particles in the solder paste; 2) the properties of the flux medium; 3) the design of the stencil to be used; 4) the paste printing parameters to be used; 5) the tendency to form voids and other defects; and 6) in sensitive devices, alpha particle emission rate.

The particle size of the suitable solder paste for a given application is limited by the minimum size of the aperture openings of the stencil to be used in printing the solder paste over the board or substrate. Excessively large particles can easily clog the stencil apertures, resulting in poor printing quality and requiring frequent cleaning that slows down production. Particle size becomes more critical as the amount of solder to be deposited becomes smaller. According to experts, the particle size of the solder paste should be no more than 12% the size of the smallest aperture opening of the stencil, i.e., at least 8 particles should be able to pass through the narrowest aperture gap at the same time.

The flux of the solder paste must have rheological properties that allow high-yield printing at very fine pitches. Of course, the flux must also exhibit excellent chemical activity for removing the thin oxide films and other contaminants from the surfaces of the metals being soldered. The flux must be easy to activate thermally, but should not decompose easily. It must also form benign residues that are quickly removed by washing.

Stencil design also impacts the effectiveness of the solder paste. As mentioned, very fine aperture openings require solder pastes with small particles. The stencil's aperture size-to-spacing ratio affects the printability of the solder paste. The shape of the aperture can also affect the size of the deposited solder for the same pitch. The stencil must be thin but rigid enough to resist deformation.

The printing parameters must also be optimized with respect to the solder paste. For instance, paste viscosity affects the speed at which printing can be done. Adequate fluidity is required to allow a good roll that fills up apertures properly. However, the paste also needs to exhibit enough stiffness to form a well-defined deposit when the stencil is separated from the board or substrate.

Pastes with the tendency to form excessive voids must be avoided. If total void elimination is impossible, voids must not be more than 5% of the solder. X-ray inspection can be used to check for voids. Lastly, experts warn about the possibility of the solder paste emitting alpha particles that can cause soft errors in memory devices, so this must be looked into if the process involves high-density memory devices.

Evaluation tests performed for selecting solder pastes include the following: 1) solder balling test; 2) wetting test; 3) solder void potential test; 4) shelf-life test; 5) tack life test; 6) stencil life and abandon time tests; and 7) slump tests. In-process evaluations must also look at the printability of the paste (relax/recovery properties, print speed, print durability), its component placement characteristics, and the quality of its solder joint/fillet formation. Solder joint reliability tests used for qualifying solder pastes include the temperature cycle test, the thermal shock test, the impact resistance test, the pressure cooker test, and the temperature-humidity-bias test.

Special solder pastes are also used for 'wafer bumping'.

Primary References: http://ap.pennnet.com; http://www.smtinfocus.com

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