Printing Solder Paste Onto PCB's - Mesh Screen and Metal Stencil Printing

Printing Solder Paste Onto PCB's

Surface mount devices (SMD's) are mounted onto a printed circuit board (PCB) by soldering their external interconnection features (such as leads, bumps, or balls) to their corresponding mounting sites on the PCB. This is achieved by depositing a material known as solder paste on the PCB's device interconnection points, positioning the surface mount devices on the board, and subjecting the board to a process known as solder reflow to melt the solder and complete the board mount process.

Solder pastes are specially blended pastes that consist of a flux medium containing graded solder powder particles. The process of depositing solder paste on the board is known as Solder Paste Printing.

Solder paste, which serves primarily as the attachment medium between the device interconnection features and the PCB itself, is deposited (usually by printing) on the attachment sites in the PCB. The device interconnections are then accurately positioned over the deposited solder paste, which is then melted (or reflowed) at a high temperature to effect the soldering of the devices to the PCB with a well-formed, contiguous fillet. After this solder reflow, the solder is allowed to cool down again to solidify and attain the mechanical properties necessary to keep the devices firmly mounted on the PCB.

There are two major process for printing solder paste onto PCB's, namely, mesh screen stencil printing and metal stencil printing. When surface mount technology first emerged in the early 80's, mesh screen printing was employed to deposit the required solder paste on the boards. Metal stencil printing was subsequently developed to replace mesh screen printing, which can not be used for smaller, finer-pitched SMD's. Mesh screen printing is still being used today because it remains to be the cheapest method, but it is applicable only to the larger SMD's that it can handle.

Figure 1. Printing solder paste on a

PCB can be a complex task

In both methods, squeegees are generally used to physically deposit and distribute the solder paste evenly across the stencil. By properly rolling the squeegee over the stencil, the solder paste passes through the stencil apertures and gets deposited on designated areas on the PCB. The stencil is then lifted, leaving behind the intended solder paste pattern on the PCB. There is, by the way, a new technique for depositing solder paste without using a squeegee. Known as direct printing, this technique employs a piston-driven printing head that presses the solder paste directly through the apertures of the stencil and onto the board.

A large variety of solder paste printing squeegees that differ in designs and material exists in the market. For metal stencil printing, thin metal squeegees are commonly used. On the other hand, mesh screen printing often employs thick rubber plates. At any rate, squeegees are generally designed to have a very smooth and non-sticking surface with a sharp printing edge.

During solder paste printing, the PCB must be held by its support in a locked position that's perfectly parallel to the stencil. The squeegee angle is usually between 45-60 degrees. A vision system is also necessary to ensure accurate printing of solder paste on the 'solder lands' of the PCB. Modern printing equipment offer many options - computer control, vision or laser print control, environment control, automatic PCB support set-up, and even stencil cleaning.

Critical parameters for high-quality solder paste printing include print speed, print pressure, separation speed/distance (or the speed/distance at which the PCB and the stencil are separated), and printer alignment. It goes without saying that excellent operator training is also imperative since solder printing is a very sensitive and delicate process.

The environment in which solder paste printing is done is also important. Defects such as solder bridging and poor wetting can be caused by dust particles or microscopic fibers in the air that end up on the PCB or stencil. Quick drying of the solder paste, on the other hand, can be caused by high ambient temperature or the presence of air draft that accelerates solvent evaporation. The viscosity of the solder paste will also be difficult to keep under control in an environment with fluctuating ambient temperature and humidity.

Primary Reference: http://www.smtinfocus.com

LINKS: Solder Paste; Solder Reflow; Solder Joint Reliability; Ball Grid Array; Lead Finish

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