Stencil Performance is on the Surface Print E-mail
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Written by Robert Dervaes   
Friday, 03 May 2013 02:38

The board is an overlooked yet important part of stencil print performance.

The most common method for determining stencil print performance for a given foil thickness has been to calculate the aspect ratio of the smallest stencil aperture (Figure 1). It compares the smallest dimension of the stencil aperture to the thickness of the metal foil. Typically, the lowest acceptable aspect ratio is 1.5.

However, advancements in stencil manufacturing technology and materials can make aspect ratios lower than 1.5 possible.

While aspect ratio is a valid determination of potential stencil performance, it is limited to simple shapes like squares, rectangles, and circles. A more accurate, and more detailed, method of determining potential stencil print performance is the surface area ratio (Figure 2). Again, it should be applied to the smallest stencil aperture. However, it can be used on any stencil aperture regardless of the shape. It compares the surface area of the stencil aperture (LxW) to the surface area of the stencil aperture walls ((2L)+(2W))x(t). Typically, the lowest acceptable surface area ratio is 0.66. However, advancements in stencil manufacturing technology and materials can make lower surface area ratios possible, with some printing extremely well below a surface area ratio of 0.5.

Both the aspect ratio and the surface area ratio are valuable formulas to use. They can help greatly in reducing some printing problems. However, component SMT pad sizes keep getting smaller and smaller. Is there anything better to use to more accurately predict stencil print performance? The answer is yes.

The stencil aspect and stencil surface area ratios only consider the stencil thickness and stencil aperture size when predicting print performance. The printed circuit board is not considered as a factor. However, the size of the SMT pad on the PCB determines the adhesion strength between the solder paste and SMT pad and is extremely important as miniature components become more mainstream. SMT pad sizes will differ from the pad sizes in the electronic PCB files (Gerber) based on the prefinished copper weights and surface finishes.

PCBs typically have a prefinished copper weight of 0.5, 1.0, or 2.0 oz. on the outerlayers. The manufacturing process consists of etching the copper to produce the circuitry (traces, SMT pads, via pads, etc.). Copper etching produces a trapezoidal cross-section (Figure 3) where the top of the SMT pad is smaller than the bottom. In most cases, the bottom of the pad will match the size in the Gerber files, since the bottom dimension is critical in meeting impedance requirements for the circuits. The top of the SMT pad will be smaller because of the copper etching process. (The size reductions can vary among PCB fabricators, but the reductions shown in Figure 3 are typical.) The smaller top size must be used in determining stencil surface area, since the adhesion of paste to pad is what pulls the paste from the stencil. As mentioned, the size of the SMT pad on the PCB determines the adhesion strength between the solder paste and SMT pad.

The most accurate method of determining potential stencil print performance is a modified surface area ratio formula that compares the surface area of the SMT pad (at the top) and the surface area of the stencil aperture walls (Figure 4). This comparison takes into account changes in the PCB pad sizes, based on copper weight and surface finish. Heavier copper weights will have a larger size difference between the top and bottom of the SMT pads. ENIG, OSP, immersion Ag, and immersion Sn surface finishes are flat and permit solder paste to stick to the entire top surface of the SMT pad. A HASL surface finish has more of a domed finish, and the semi-rounded surface makes it more difficult for the solder paste to stick to the entire surface. This will reduce the adhesion strength between the solder and pad. Heavier copper boards, and those with HASL surface finishes, will present the most print challenges when miniature components are present.

Stencil aperture sizes have been increased many times with the expectation that a larger stencil aperture size increases the surface area ratio. In many cases, solder paste release was not improved, due to the fact that the size of the SMT pad, and the corresponding adhesion strength between the solder paste and SMT pad, did not change. There are times when increasing the stencil aperture size can help, however. Provided the modified surface area ratio passes, the benefit of a larger stencil aperture size is a wider alignment tolerance between stencil and PCB at the printer. The wider alignment tolerance increases the process window at the printer and can increase assembly yields.

Robert Dervaes is vice president of technology at FCT Assembly (fctassembly.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Last Updated on Friday, 03 May 2013 14:42
 

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