亲爱的罗恩博士
We are experiencing some graping on BGA solder joints. One of my colleagues said he heard that using square versus our current round stencil apertures might solve the problem. This seems fishy to me, since the amount of solder paste difference isn't that great, I'm surprised that it could make that much difference. Can you shed some light on this issue?
预先表示感谢、
埃里克
亲爱的埃里克
你的朋友可能是对的。尽管圆形孔径和方形孔径的体积相差不大，但计算表明，圆形孔径的体积是方形孔径的 0.7854。因此，方形孔径的体积只比圆形孔径多出 20% 多一点。见图 1。
图 1.圆形孔径的体积仅为方形孔径的 78.54%。
The circular aperture is only 78.54% of the volume provided by a square aperture. However, experiments have shown that square apertures provide up to 40% more solder paste than round apertures. Discussions I have had with people that are knowledgeable in stencil printing suggest that the curvature of the circular aperture provides more contact to the solder particles than a square aperture. See Figure 2. This increased contact could result in more adhesion of solder particles to the aperture, resulting in less transfer efficiency.
Figure 2. The curved surface of the circular aperture provides more contact area for solder particles to adhere to, hence reducing solder transfer efficiency.
The curved surface of the circular aperture provides more contact area for solder particles to adhere, hence reducing solder transfer efficiency. Graping is caused by the oxidation of solder paste particles. So, if square apertures provide more solder paste, how can this reduce graping? Solder paste deposits today are very tiny compared to what they were just a few years ago. These small deposits have a large surface area to volume ratio. This situation requires the oxygen barrier components of the solder paste flux to work overtime to protect the solder particles from oxidation during reflow. A larger solder paste deposit, such as provided by the square aperture, minimizes this oxidation risk. This effect can be seen experimentally in Figure 3.
Figure 3. The square aperture eliminates graping as compared to the circular aperture.
The square aperture eliminates graping as compared to the circular aperture. It should be pointed out that modern solder pastes have also be designed to minimize graping. So, working with your solder paste vendor to solve a graping problem is also a good idea.
干杯，罗恩博士
