As the density of components in electronic systems increases, designers often choose equivalent precision connectors with a coplanarity of no more than 0.10 mm to match a 0.10 mm thick solder paste stencil on a printed circuit board (PCB). . However, there are many connectors with a coplanarity value of 0.15 mm on the market, and connectors with a coplanarity value of 0.10 mm are also difficult due to the increase in the number of pins and the introduction of special pins and right-angle connectors. getting bigger. This therefore limits the designer's connector choices; either has to use multiple connectors when a single connector could be preferred, or is forced to use a stepped solder paste stencil. Both options add cost and complexity to system design and production.

However, a study by Samtec Inc. and Phoenix Contact shows that by optimizing the opening shape of the solder paste stencil, designers can The finer 0.10 mm solder paste stencil is used in conjunction with the IPC-J-STD-001 Class 2 standard with 100% yield.

This article will discuss the relationship between solder paste stencils and connector coplanarity, as well as topics such as the trade-offs and constraints faced by designers. The paper then presents the status of this study and the corresponding results, and the impact these results have on cost, space, performance, and reliability when optimizing designs.

Relationship Between Solder Paste Stencils and Connector Coplanarity

Precisely applying a small patch of solder paste is not too difficult with precision machined solder paste stencils. However, as the number of connector pins continues to increase, some pins on the connector need to be made into specific shapes and made into specific connection types such as right-angle connections. There is increasing matching difficulty between the solders. The main problem is due to the coplanarity of the connector pins.

Simply put, the term "coplanarity" refers to the maximum distance between the tallest and lowest leads (or pins) when a connector is placed on a flat surface. The value of this distance is usually measured with optical measuring equipment (Figure 1, left).

Figure 1: Coplanarity is the maximum difference between the heights of different leads measured on a plane; minimizing this difference is critical for the leads of surface mount (SMT) devices. This avoids problems with solder joints. (bottom right).

Good coplanarity is critical for good solder joints: if a pin or lead is positioned too high, it may not make adequate contact with the solder paste, resulting in a mechanically weak or full solder joint. Leak soldering with open electrical connections. Most specifications call for coplanarity between 0.10mm and 0.15mm.

By utilizing the right processes and tools, it is possible to consistently build connectors with 0.15 mm coplanarity for most applications. However, due to the increase in the number of pins, especially the pins of some connectors have developed into a specific shape, or they need to be connected at a specific angle (eg double row, right angle) to achieve a coplanarity of 0.10 mm more difficult. Maintaining this low coplanarity increases connector cost.

Today's large circuit boards contain more than 3000 components and smaller, more highly integrated electronic devices, making the already tight board space even more dense. As a result, the spacing between component pins is also Getting smaller, designers are now considering more solder paste stencils with a thickness of 0.10 mm. If thicker solder paste stencils are used, there is a higher risk of solder bridging between leads or pads. However, it is difficult for designers to find connectors that meet the 0.10mm coplanarity specification while still having a sufficient pin count and form factor.

Of course, designers do have other solutions as well. For example, they can use stepped solder paste stencils, with thinner stencils for fine-pitch components and larger stencils for connectors. This solves the problem, but the cost of the solder paste stencil becomes higher, and it may not be suitable for applications where there is insufficient space between components on either side of the solder ladder. As a general rule of thumb, the distance between the two step openings should be 36 times the step thickness.

Another option is to use multiple connectors. The lower the pin count of a connector, the easier it is to meet tighter coplanarity specifications. But multiple connectors add cost, layout complexity and reliability issues. Additionally, while the connector may meet the 0.10 mm coplanarity requirement, a 0.10 mm thick solder paste stencil implies a lower solder height, which may result in a joint that may not be mechanically strong enough.

How to Optimize the Opening of Solder Paste Stencils

To minimize these tradeoffs, Samtec and Phoenix Contact investigated the effects of modifying the solder paste stencil openings for three series of connectors. These studies used a stencil with a thickness of 0.15 mm and a 1:1 opening to allow the deposited solder to conform to the size and shape of the copper pads. Two templates with a thickness of 0.10 mm but larger openings were subsequently added to these experiments, and connectors with coplanarity in the range of 0.10 mm and 0.15 mm were fabricated and selected in subsequent studies.

The study involved overprinting by sizing the solder paste stencil openings beyond the pad size to increase solder volume and make better connections, but not enough to cause solder bridges or leave solder balls on the board surface. To achieve this, the study relied on the tendency of the solder paste to form condensation on the heated pads after reaching its liquefaction temperature during reflow. Of course, the correct cutout size must be determined for each connector type (Figure 2).

Figure 2: The orange outline shows the optimum cutout size for FTSH connectors.

For example, to ensure a good solder joint between a 0.152 mm coplanar FTSH connector sample and a 0.10 mm thick solder paste stencil, the optimal opening is 2.84 mm x 0.97 mm. This makes it possible to achieve high-quality solder joints that meet the requirements of the IPC-J-STD-001 Class 2 standard for 100% yield (Figure 3).

Figure 3: Soldering results of a sample FTSH connector with 0.152 mm coplanarity using a 0.10 mm thick solder paste stencil with optimized openings showing the inner row (left) of the pins and The outer row of pins (pictured to the right) are all high-quality solder joints.

Based on these results, it is clear that designers should reconsider connectors with a maximum coplanarity value of 0.15mm when using a 0.10mm thick paste stencil. Once the optimal template cutout has been determined to support the combined mode, the many off-the-shelf connectors can be used to expand the choice and avoid the limited range of expensive alternatives. If the optimal aperture is not available online or has not been determined, it is important to contact the connector manufacturer early in the design process to determine the optimal aperture, or find a more suitable aperture for any identified application solution.

Early intervention is critical, and as the design process progresses, the options become narrower.

in conclusion

Fully understanding the trade-offs and hearing customers' desire for more precise solder paste stencils and tighter coplanarity, Samtec Inc. and Phoenix Contact's R&D teams worked together to develop an optimized solder paste stencil development. A hole approach, which allows a 0.15mm coplanar connector to be used with a 0.10mm thick solder paste stencil. This research has resulted in the world's best results: precision solder paste stencils with a thickness of 0.10mm, greater connector selection, low cost, low complexity, mechanical strength in IPC-J-STD-001 Class 2 required high quality solder joints.