The Area Ratio for Odd-Shaped Stencil Apertures

Joey writes:

Dear Dr. Ron,

I have a stencil aperture with an unusual shape. See Figure 1. How do I calculate the area ratio? The stencil thickness is 5 mils. The dimensions of the aperture are also in mils.

Figure 1. Joey’s Stencil Aperture


The area ratio is simply the area of the stencil aperture opening divided by the area of the sidewalls. For common aperture geometries such as circles, squares, etc. it is easy to derive formulas. See Figure 2.

Figure 2. Formulas can be developed for common aperture shapes.

For an unusual shape like yours, it is easiest to simply calculate and divide the areas. From Figure 1, we get that area of the aperture opening as: 40*24+ the area of the two triangles. A little geometry (can you do it?) shows each triangle to have an area of 89 sq mils. So, the total area is 960 + 2*89 = 1138 sq mils. The perimeter is 40+24+16+16+28+12+16+16 = 168 mils, hence the area of the sidewalls is 168*5 = 840 sq mils. Therefore, the area ratio is 1138/840 = 1.355. Experience has shown that an area ratio of > 0.66 is needed for good solder paste transfer efficiency, so this stencil aperture will do well for transfer efficiency.

Careful thought would suggest that the triangular protrusions alone do not have a good area ratio. Calculations show their area ratios to be 0.37. So, the transfer efficiency in this part of the aperture might not be good. However, the area of the rectangle is so great, more than five times that of the triangles, as to alleviate this concern.

Dr. Ron

GKG: Westward Ho?

Southeast Asian assembly process equipment companies have approached Western markets in fits and starts.

A few have made inroads: From time to time, we have seen JT and Fulongwin soldering equipment at US plants, usually smaller ones (Flextronics is an exception) and often on the US West Coast. But while we’ve been reporting for more than a decade on the availability of literally scores of Chinese-made brands, some of which are very popular in Taiwan and China, it’s still highly unusual to see any make it across the ocean.

Many have been stymied by patent issues that effectively have blocked them specifically from the US and European markets. Another problem is finding good channel partners. From time to time, firms ranging from independent reps like FHP Reps and Bill West to solder paste vendors like Qualitek have tried, with limited success. Service and access to spare parts have been limiting factors.

That’s what makes Friday’s announcement from GKG so interesting. GKG has named Juki as exclusive distributor of its screen printers in the Americas. Known primarily for its placement equipment, Juki has been inching toward a full-line offering for the past couple years, having begun distributing Intertec’s selective soldering equipment in 2009.

For years, DEK and Speedline have dominated the Western printer markets, with Asys/Ekra in third with an estimated 10% share. Juki’s track record and never-say-die approach to selling makes it a formidable competitor. However, Juki has many of the same distributors as Speedline, and it is unclear that they will give up the latter for a new player.

But the real prize may be the emerging South America market. As Juki CEO Bob Black told us, “In Latin America, out major competitors are offering complete lines. To be competitive, we need to do the same.” And Juki has the breadth and depth in its service department that many standalone reps have not.

Keep an eye on this.