Components can be kept in a precise location during and after reflow with the proper application of capillary action.
Janos Legrady
Sharon Harvey
Originally, the first surface mount technology (SMT) terminal was developed as a test point, where the locational tolerances and perpendicularity were not critical factors (see Figure 1). Actual testing, however, showed that terminal location and perpendicularity could be held to very tight tolerances. The product, therefore, lent itself for use as a 0.025" square pin.
The terminal consists of a 0.025" square body, 0.375" long with a 0.075" square base perpendicular to the body. The part is stamped from a 0.012" thick brass strip and folded to create the 0.025" square body. The resulting space between the two halves of the pin`s body acts as a narrow tube into which the melted solder flows, providing the capillary action.
Surface Mount Pins
One of the best ways to demonstrate the effects of capillary action is by using a surface mount pin. Two similar pins are compared, the only difference between them being that one has a small capillary tube at the center and the other does not (see Figures 2 and 3).
Both pins are placed on 0.010" thick solder paste printed on a copper solder pad. Figures 2 and 3 show the pins just before solder reflow. They are tower-like structures with small bases and high centers of gravity. They sit in the solder, perpendicular to the printed circuit board (PCB).
During reflow when the solder becomes liquid (before solidifying), the solid pin without the capillary tube floats on the top of the melted solder. The melted solder becomes somewhat dome-shaped, and the pin is perpendicular to the surface of the solder dome, but not necessarily to the surface of the PCB. The degree of the pin`s perpendicularity to the PCB depends on its location on the solder dome. In experiments using pins with the dimensions shown in Figure 2, less than 10 percent were perpendicular to the PCB.
If the pin has a capillary tube, the melted solder rises in the tube. While the solder is being pulled up into the capillary tube from under the terminal, the same force concurrently pulls the pin down to the surface of the solder pad. This pin does not float on top of the melted solder and is always perpendicular to the PCB. Once the solder solidifies, the solid pin without the capillary tube stands on top of the solder dome, but is not perpendicular to the PCB.
Pins that exhibit capillary action sit directly on the copper solder pad. There is only about a 0.002" thick layer of solder between the base of the pin and the solder pad. The postreflow location of the pin on the PCB is exactly the same as it was before reflow because it does not float on top of the melted solder, but instead is pulled down to the surface of the solder pad by the capillary action.
Pull-force tests reveal that a pin with proper capillary action has a much higher retention to the PCB than one without the capillary action feature. This higher retention force is attributed to two things. First, there is only a thin layer of solder between the base of the pin and the solder pad. Solder is a weak alloy with a low yield stress. A thicker layer of solder will fail before a thinner layer. This fact becomes more apparent if one thinks of solder as behaving in the same way as an adhesive. It is undeniable that a thinner layer of adhesive provides a stronger bond than a thicker layer.
Second, as the solder reflows, there is a certain amount of outgassing because of flux and other active ingredients in the solder. These gases get trapped under a relatively large surface like the base of the pin and create voids in the solder, which are clearly visible when the pin is pulled off or the solder joint is cross-sectioned. Pins with capillary action have fewer and smaller voids because the capillary tube provides a way for the gases to escape. Capillary action-enhanced pins are more resistant to the effects of thermal cycling because cracks in solder joints grow from such gas-created voids.
Feeders
Because no fixturing is needed, it is possible to place the capillary action-enhanced SMT pins and other terminals the same way as all the other SMT components being placed. The only thing needed is a special feeder that can fit in the standard feeder slot of the applicable placement system. The feeder consists of a mechanism that unreels and guides the continuous strip of terminals. The shear/clamp mechanism shears off one single terminal from the strip, holds it in a precise pick-up position and releases the terminal when it makes contact with the vacuum pick-up nozzle. Feeders eliminate the need for taping of parts, which provides substantial cost savings because the taping of surface mount terminals often costs more than the terminal itself.
Pin Headers
Another benefit of capillary action-enhanced components is that they can eliminate problems associated with coplanarity and PCB warpage. This is best demonstrated by using an SMT header as an example. The header assembly consists of square pins that are pressfit into round, plated through holes on a "header PCB" (see Figure 4). The end of the pin is flush with the bottom of the header PCB. The plated through holes are located at the center of a square solder pad on the bottom of the board. On the topside of the board, there is a small, circular solder pad centralized around the plated through hole. Both the bottom and top solder pads are connected to the conductive wall of the plated through hole.
The size of the hole is such that it holds the square pressfit pin firmly in place, but allows for four cavities on the four sides of the pin. The cavities are defined by the flat side of the pin and the curved wall of the plated through hole. The function of the cavities is to promote capillary action. Deep score lines run across both the top and bottom of the header PCB, making the board very flexible. The PCB that receives the header assembly must have surface mount solder pads of the same size located at the same pitch as the solder pads on the bottomside of the surface mount header assembly. The solder is stenciled over the pads on the PCB, and the header assembly is placed over the solder in such a way that the solder pads on the receiving PCB and on the bottom of the assembly align.
When the solder heats and reflows in the oven, the capillary action provided by the four cavities around the pin sucks up most of the melted solder and solders the pin into the plated through hole at the same time it solders the header assembly to the PCB. The force that sucks the melted solder into the cavities also pulls the header assembly and the PCB tightly together. Because the score lines make the PCB header assembly very flexible, it conforms to the shape of the PCB even if either or both boards are warped
Capillary action forces provide two additional benefits: they pull the header to the right position to be aligned with the solder pad even if it is placed off-center, and they produce a much stronger solder joint between the header and the PCB. Because solder is a weak alloy, it is preferable to have the minimum amount of solder between the two surfaces being attached. The capillary action siphons away all the excess solder, thereby making the solder joint much stronger. The melted solder runs up the plated through hole to the topside of the header assembly board and forms a ring (fillet) around the pin. This indicates that the reflow process is complete and ample solder was used (see Figure 5).
Conclusion
Components that would float or fall over during reflow can be kept in a precise location during and after reflow with the proper application of capillary action. The resulting solder joint is also stronger and withstands more thermal shocks and thermal cycles. When there are a number of solder joints on a component, the capillary force pulls a semi-flexible assembly together, eliminating the issues associated with coplanarity problems and PCB warpage. Also, if there are several solder pads on a component, this same force centers the assembly over its correlating pads even if it was not aligned precisely before reflow. In general, the capillary action-enhanced solder joint is more robust, reliable and forgiving in surface mount assembly.
Adapted from a paper originally published in the IICIT Annual Symposium Proceedings, September 27-29, 1999.
JANOS LEGRADY is manager of research and development, and SHARON HARVEY is research and development project manager, Zierick Mfg. Corp., Radio Circle, Mt. Kisco, NY 10549; (914) 666-2911; Fax: (914) 666-0216; Web site: www.zierick.com.
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Figure 1. Pins soldered onto a PCB standing perpendicular.
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Figure 2. A solid pin without the capillary tube. The circle with a slash shows diameter.
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Figure 3. A pin with the capillary tube going all the way through the center.
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Figure 4. A 3-D view of a header assembly.
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Figure 5. Cross section of the SMT header. The force that sucks the melted solder into the cavities also pulls the header board assembly and the PCB together. The ring at the top indicates that the reflow process is complete.
SPEC SHEET
End Applications: PCBs with surface mount components
Related Products: SMT connectors, PCBs, pins, pin headers, solder
Main Point: These surface mount terminals take advantage of melted solder`s interaction with small capillaries. Capillary action-enhanced surface mount components have a stronger solder joint and reduce the incidence of solder joint fracture. Capillary action is an effective means of holding the precise location of components on the PCB during and after reflow. It also minimizes board warpage and coplanarity problems.
