Expand the range of connector options by optimizing the solder paste template opening

With the increase in the density of components in Electronic systems, designers usually choose connectors with the same precision and coplanarity not exceeding 0.10 mm in order to match the solder paste template with a thickness of 0.10 mm on the 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-shaped pins and right-angle connectors. getting bigger.This therefore limits the designer’s range of connector choices; or when a single connector could be used first, they have to use multiple connectors, or are forced to use step welding.

In high-density electronic systems, by carefully designing the shape of the openings, designers can now match a connector with a coplanarity of 0.15 mm and a solder paste template with a thickness of 0.10 mm.

Author: David Decker, Director of Interconnect Technology, Samtec Inc.

With the increase in the density of components in electronic systems, designers usually choose connectors with the same precision and coplanarity not exceeding 0.10 mm in order to match the solder paste template with a thickness of 0.10 mm on the 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-shaped pins and right-angle connectors. getting bigger. This therefore limits the designer’s range of connector choices; or when a single connector could be used first, they have to use multiple connectors, or are forced to use a stepped solder paste template. Both of these two options increase the cost and complexity of system design and production.

However, a study by Samtec Inc. and Phoenix Contact showed that by optimizing the hole shape of the solder paste template, designers can choose the widely available, lower-priced, 0.15 mm coplanarity connector to interact with The finer 0.10 mm solder paste template is used together, and it can also meet the requirements of IPC-J-STD-001 Class 2 under the condition of 100% yield.

This article will discuss the relationship between solder paste templates and connector coplanarity, as well as topics such as trade-offs and constraints faced by designers. Then this article will introduce the status of this research and the corresponding results, as well as the impact of these results on cost, space, performance and reliability when optimizing the design.

The relationship between solder paste template and connector coplanarity

It is not too difficult to accurately apply a small piece of solder paste using a precision-machined solder paste template. However, as the number of connector pins continues to increase, at the same time, some pins on the connector need to be made into specific shapes and specific connection types such as right-angle connections. The connector and the use of precision solder paste templates are used to form There are increasing difficulties in matching between the solders. The main problem is caused by the coplanarity of the connector pins.

In short, the term “coplanarity” refers to the maximum distance between the highest and lowest height leads (or pins) when the connector is placed on a flat surface. The value of this distance can usually be measured with optical measuring equipment (Figure 1, left).

Expand the range of connector options by optimizing the solder paste template opening

Figure 1: Coplanarity refers to the maximum difference between different lead heights measured on a plane; for the leads of surface mount (SMT) devices, it is important to minimize this difference. This can avoid problems with solder joints. (Bottom right corner). (Image source: Samtec Inc.)

Good coplanarity is essential for good solder joints: if a pin or lead is positioned too high, it may not be able to form sufficient contact with the solder paste, which may result in solder joints being mechanically soldered or completely soldered. Missing welding of an open electrical connection. Most specifications require that the coplanarity be between 0.10mm and 0.15mm.

By using the right process and tools, it is possible to continuously build connectors with a coplanarity of 0.15 mm for most applications. However, due to the increase in the number of pins, especially when the pins of some connectors have developed into specific shapes, or they need to be connected at a specific angle (such as double row, right angle), a coplanarity of 0.10 mm must be achieved It’s even more difficult. Maintaining this low coplanarity increases the cost of the connector.

Today’s large circuit boards include more than 3,000 components and smaller and more integrated electronic devices, making the already tight board space denser. As a result, the spacing between the component pins is also It is getting smaller and smaller, and designers are now thinking more about using solder paste templates with a thickness of 0.10 mm. If a thicker solder paste template is used, there will be a higher risk of solder bridges between leads or pads. However, it is difficult for designers to find a connector that not only meets the 0.10mm coplanarity specification, but also has sufficient pin counts and suitable external dimensions.

Of course, designers can indeed choose other solutions. For example, they can use a stepped solder paste template, use a thinner solder paste template to deal with small pitch components, and use a larger template to support the connector. This solves the problem, but the cost of the solder paste template will become higher, and it may not be suitable for applications where the space between the components on both sides of the solder ladder is insufficient. According to general experience, the distance between the two step openings should be 36 times the step thickness.

Another option is to use multiple connectors. The fewer the number of pins in the connector, the easier it is to meet the tighter coplanarity specifications. However, multiple connectors increase the cost, and at the same time increase the complexity of the layout and bring reliability problems. In addition, although the connector may meet the 0.10 mm coplanarity requirement, a solder paste template with a thickness of 0.10 mm means a lower solder height, which may result in insufficient mechanical strength of the solder joints.

How to optimize the opening of the solder paste template

In order to minimize these compromises, Samtec and Phoenix Contact studied the effects of modifying the solder paste template openings for three series of connectors. These studies used a template with a thickness of 0.15 mm and a 1:1 opening to make the size and shape of the deposited solder consistent with the copper pad. Later, in these experiments, two templates with a thickness of 0.10 mm but with larger openings were added, and connectors with coplanarity in the range of 0.10 mm and 0.15 mm were fabricated and selected in the following research.

This research involves adjusting the size of the solder paste template opening to exceed the pad size for overprinting to increase the amount of solder and form a better connection, but not so much as to cause solder bridges or leave solder balls on the surface of the circuit board. To achieve this goal, this research relied on the tendency of the solder paste in the reflow process to condense on the heated pad after reaching its liquefaction temperature. Of course, the correct opening size must be determined for each connector type (Figure 2).

Expand the range of connector options by optimizing the solder paste template opening

Figure 2: The orange outline shows the optimal opening size of the FTSH connector. (Image source: Samtec Inc.)

For example, to ensure that a good solder joint is formed between an FTSH connector sample with a coplanarity of 0.152 mm and a solder paste template with a thickness of 0.10 mm, the best opening is 2.84 mm x 0.97 mm. In this way, high-quality solder joints that meet the requirements of the IPC-J-STD-001 Class 2 standard can be realized for 100% yield (Figure 3).

Expand the range of connector options by optimizing the solder paste template opening

Figure 3: The soldering result of an FTSH connector sample with a coplanarity of 0.152 mm after using a solder paste template with a thickness of 0.10 mm and an optimized opening. You can see the pins and pins in the inner row (left picture) The outer row pins (pictured on the right) are all high-quality solder joints. (Image source: Samtec Inc)

Based on these results, it can be clearly seen that when designers use solder paste templates with a thickness of 0.10 mm, they should again consider using a connector with a maximum coplanarity of 0.15 mm. If it has been determined that the best template opening is used to support the combination mode, a large number of ready-to-use connectors can be used to expand the range of options and avoid the use of expensive alternatives within the restricted range. If the best opening is not available online or has not been determined, it is important to contact the connector manufacturer at the beginning of the design process to determine the best opening, or to find a more suitable opening for any identified application solution.

Early intervention is critical. As the design process deepens, the choices become narrower and narrower.

in conclusion

With a full understanding of various trade-offs and hearing customers’ desire for more precise solder paste templates and tighter coplanarity, the R&D teams of Samtec Inc. and Phoenix Contact have jointly developed an optimized solder paste template. Hole approach, it enables connectors with a coplanarity of 0.15mm to be used in conjunction with a solder paste template with a thickness of 0.10mm. This research has brought the best results in the world: precision solder paste templates with a thickness of 0.10mm, a larger range of connector options, low cost, low complexity, and mechanical strength that meets the IPC-J-STD-001 Class 2 standard The required high-quality solder joints.

About the author:

David Decker, Director of Interconnect Technology, Samtec Inc.

www.samtec.com

Expand the range of connector options by optimizing the solder paste template opening

David Decker received a master’s degree in mechanical engineering from the University of Louisville’s Speed ​​Scientific School in 1993 and a professional engineering license in 1998. He started his career as an injection mold engineer at Lexmark, Inc. and then worked in General Electric Appliances. Later, David joined Samtec and worked for the company for 22 years. He has held important positions in new product design, customized product design and other departments, and has been the director of the interconnection process department for the past 15 years. David also served as a lieutenant in the Reserve Department of the Clark County Sheriff’s Office and served there for 9 years.

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