To achieve accurate positioning of components, surface mount assembly PCBs must be designed to ensure that component pad (land pattern) dimensions and layout are in accordance with good practice and the component manufacturer’s recommendation. For instance, where a component is using two pads, and one is larger than the other, the larger pad, will exert higher surface tension forces while in the liquid state and will pull the component off the smaller pad or raise it in the air on the larger pad. This failure mode is referred to as ‘Tomb-stoning’ because of the similar appearance to tombstoning. This problem also occurs where the heat sinking of one pad is much greater than the other, causing unequal solidification times for both ends of the component. Such failure modes are often regarded as a component solder ability issue.
On a surface mount assembly, components are attached to the PCB using solder paste, which is a combination of solder balls (powder) and a compound containing binder flux and solvents, in roughly equal volumes. The consistency is similar to tooth paste. Solder balls are specified in many grades and sizes depending on the application and component lead pitch i.e. fine pitch 0.4 mm lead frame terminations would dictate a small dimension solder ball. Figure 8-3 shows an X-ray image of three solder paste samples, each consisting of different solder ball dimensions. Solder paste may be applied to the PCB manually via a syringe at each individual pad or, more commonly automatically by stencil printing. Stencil paste application is significantly faster and is achieved using a metal stencil and squeegee blade. A metal stencil is produced with apertures that align exactly with the printed circuit board land pattern layout.
The stencil is loaded with solder paste and positioned over the PCB and the solder paste forced through the apertures using a squeegee blade action. The angle of the blade is critical, and is adjusted depending on the printing density of the board.
Figure 8-4 illustrates this process where the stencil, loaded with solder paste, is lowered and located accurately over the PCB. The squeegee is then pulled across the stencil forcing paste through the apertures and onto the component land pattern. The stencil and apparatus are then raised and ready for the next PCB to automatically line up below it.
The stencil thickness and aperture dimensions, control the volume of paste deposited on the PCB Stencil screen preparation and production is a high precision process, and is the only way to meet the printing tolerances required for fine lead pitch, chip scale and ball grid array (BGA) devices. High-density printing is normally associated with thinner paste deposits and finer solder paste metal (tin/lead) particles. Typical paste deposit thickness range from 150 microns to 250 microns.
