Overcome the challenges of alignment of multiple connector groups between PCBs

Printed circuit board (PCB) manufacturers are under significant pressure to increase density, shrink footprint, reduce side size, manage heat flow, and increase data rates while increasing reliability and reducing cost. As they continue to successfully reduce these pressures, designers face an interesting challenge of aligning multiple mated connector sets between two PCB boards.

Miniaturization has made alignment of multiple connectors more difficult, and the pursuit of optimal design practices and early engagement with connector providers can help ensure design success

By Kevin Meredith, Product Engineer, Samtec

Printed circuit board (PCB) manufacturers are under significant pressure to increase density, shrink footprint, reduce side size, manage heat flow, and increase data rates while increasing reliability and reducing cost. As they continue to successfully reduce these pressures, designers face an interesting challenge of aligning multiple mated connector sets between two PCB boards.

What is needed are clear guidelines for understanding how to address these alignment challenges without sacrificing system performance, density and reliability, while meeting increasingly stringent budget and time-to-market requirements.

This article discusses the challenges of alignment in more detail before describing the conflicting requirements that may be encountered between advanced PCBs and more reliable high-density connectors so that these requirements can be efficiently met through the use of design best practices.

Miniaturization makes connector alignment difficult

There are many areas in which PCB boards can be improved, including density, higher data rates, thermal management and reliability. Accompanying these improvements, however, is the trend toward miniaturization that puts pressure on designers in terms of connector selection and implementation, especially for mating multiple connectors onto a PCB board.

In the case of connectors, miniaturization over the past 25 years has resulted in pitches dropping from 0.100 inches (2.54 mm) to 0.016 inches (0.40 mm) — a sixfold reduction, requiring tighter tolerances. However, tighter tolerances are not an issue per se, the problem is the variability around the nominal tolerance: if multiple connectors go to either limit of the nominal, some problems are more likely.

Applications with a single mating connector set do not present a problem: because there is no tolerance build-up, the mezzanine card is assumed to be free-floating, and the global and local alignment features of the connectors will ensure perfect alignment (Figure 1, top).

Overcome the challenges of alignment of multiple connector groups between PCBs

Figure 1: Applications using a single mating connector (top) have no stacking tolerances, and the connector’s global and local alignment capabilities will ensure perfect alignment. Multiple connectors introduce tolerances that add up and cause alignment errors. (Image credit: Samtec Inc.)

However, adding more mating connector sets in any direction and any distance to the same female mezzanine card will introduce some cumulative tolerance (Figure 2, bottom). These tolerances are especially important for PCB fabrication shops, Electronic manufacturing services, and the properties of materials used in PCB boards.

To illustrate this point, consider a multi-mezzanine connector system (Figure 2). This assembly includes six or more components: motherboard (A), mezzanine card (B), female connector #1 (C), with mating male connector #1 (D), female connector # 2 (E) mates with male connector #2 (F).

Overcome the challenges of alignment of multiple connector groups between PCBs

Figure 2: Designers need to consider and account for all component tolerances, including the PCB board.

(Image credit: Samtec Inc.)

Provided that a mezzanine connector and a sufficiently rigid PCB can be manufactured, machined, and assembled precisely to nominal conditions, an infinite number of connectors can be successfully deployed between two PCBs; in fact, tolerances and material properties can vary. Transgender is the limiting or decisive factor. In the case shown in Figure 2, the designer needs to consider and account for tolerances for all components, including (A) and (B) two of the often overlooked but relevant tolerances of the PCB.

How to Solve PCB Board to Connector Alignment Problems

The procurement of some PCB boards is only governed by the specifications embedded in the Gerber data package (Figure 3). PCB boards can be built from these packages without regard to mechanical tolerances.

Overcome the challenges of alignment of multiple connector groups between PCBs

Figure 3: The procurement of some PCB boards is only governed by the specifications embedded in the Gerber data package, so that the PCB can be built according to these data packages without regard to mechanical tolerances. For multi-connector applications, this data package needs to be accompanied by a separate mechanical drawing.

(Image credit: Samtec Inc.)

For multi-connector applications, this data package must be accompanied by a separate mechanical drawing indicating the original drawing, drilling and routing tolerances.

At this point, designers need to do two things to help ensure a successful outcome. The first is to understand what support PCB board suppliers and connector suppliers can provide to ensure alignment. The second is to ensure that a system-level tolerance study has been conducted to determine the connector alignment deviations that result from its design.

Looking back at the multi-connector mezzanine card system of components A through F in Figure 2, the connector supplier can only control the tolerances of the connectors. A good supplier will meet or exceed published performance specifications, make PCB board tolerances and processing recommendations, and even provide reference recommendations for recommended PCB suppliers and equipment as needed.

System or product designers should refer to the connector footprint and product specifications. The alignment deviation specifications contained in these documents should be compared with the results of system-level tolerance studies to help ensure that multiple connectors between the same boards are used successfully.

The connector system will function properly as long as the initial and final angular and linear alignment deviations are not exceeded. These alignment deviation values ​​are calculated by taking into account factors such as insulator interference, beam deflection, and contact friction. Exceeding the misalignment value may result in open or damaged circuits and/or insulators.

While all the necessary information on design, component tolerances, equipment and manufacturing capabilities are often readily available to designers, it is important to be able to get in touch with the connector manufacturer to provide more specific guidance and verification of alignment deviation tolerance accumulation .

Dowel pins are not suitable for multi-connector applications

Some connector manufacturers offer optional alignment pins, which are usually located on opposite sides of the bottom of the connector (Figure 4). These locating pins facilitate manual placement, can be used to help orient the connector on the PCB, and for single-connector applications, they do not add to the overall tolerance buildup.

Overcome the challenges of alignment of multiple connector groups between PCBs

Figure 4: While locating pins are useful for both manual placement and orientation, they are not recommended for multi-connector applications due to their impact on overall tolerance build-up.

(Image credit: Samtec Inc.)

However, for multi-connector applications, we do not recommend the use of dowel pins because of their impact on the build-up of overall tolerances. If orientation on the PCB is still required, a better option is to drill some oversized holes in the PCB and use a machine to place the connectors.

Likewise, the use of clamps or pins to aid connector placement is not recommended. These methods typically rely on drilling holes on the PCB relative to the original, but the positional tolerance of that hole is often poor relative to another connector, which reduces the overall accuracy of the final connector placement.

For multi-connector applications, a better approach is to position all pads starting at position A1 in the solder pad array A, and then precisely place the connectors on the pads before reflow.

Fix the PCB board with fastening screws

Some particularly rugged applications may require the use of captive screws to protect both PCB boards. In this case, the screws should be as close as possible to the connector system (Figure 5).

Overcome the challenges of alignment of multiple connector groups between PCBs

Figure 5: If captive screws are to be used, they should be placed as close as possible to the connector system.

(Image credit: Samtec Inc.)

Placing them close together will concentrate stress near the connector and reduce unsupported PCB board span. The increase in span can cause bending stress in the PCB board, which can adversely affect other components, especially surface-mounted components.

Another source of connector-derived PCB board stress is the loading process, where a large amount of insertion and extraction (I/O) creates very high insertion/extraction forces. These forces can cause excessive deflection of the PCB so that additional stiffeners are required to support the PCB. It is very important to confirm the insertion force and extraction force of the connector, which can be found in the product quality inspection test report. (Image 6).

Overcome the challenges of alignment of multiple connector groups between PCBs

Figure 6: To avoid out-of-spec loading, the designer should always verify the insertion and extraction force of the connector, which can be found in the product quality test report.

(Image credit: Samtec Inc.)

in conclusion

Although the miniaturization trend makes it more challenging, it is possible to use multiple connector sets on two PCB boards by using best design practices. These include conducting a system tolerance study to determine connector alignment deviations, then following the connector provider’s recommended footprint and die design, and utilizing machines to place components.

Also, it is advisable to work closely with the connector provider early in the design process, as they can advise on connector type and placement, as well as provide advice on how to minimize overall PCB and connector stress to help to ensure the success of the design.

About the author:

Overcome the challenges of alignment of multiple connector groups between PCBs

Kevin Meredith has 27 years of design and industry experience in the connector industry. Kevin is currently working as a product engineer at Samtec Inc., having previously held positions at 3M and Robinson Nugent. He is a mechanical engineer and a graduate of the JB Speed ​​School of Engineering at the University of Louisville.

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