Tolerance Stack Up in Multi-Connector Applications 

Mating a daughtercard (sometimes called a daughterboard) in a mezzanine application to a motherboard using two or more high-density array connectors or multiple fine-pitch connectors is quite common in complex system designs today. To ensure proper end system functionality, it is good practice to perform a tolerance stack-up analysis long before manufacturing begins. 

Importance of Tolerance Stack-up Analysis 

Let’s first examine a single connector mezzanine application. In such an application the daughtercard is assumed to be free floating with alignment features within the connectors themselves ensuring perfect alignment, therefore being unimpacted by PCB fabrication and assembly process tolerances.  

However, in a multi-connector application with two or more connectors, mechanical tolerances will stack in any direction at any distance. Tolerances from the board house for motherboard and daughtercards, the equipment, and final assembly process all play a role in cumulative tolerance stacking and can impact connector functionality.

Predictive analysis early in design stages will ensure final system reliability. 

Considerations & Recommendations 

From an interconnect perspective, the best starting point when examining tolerance stacking is to confirm final placement of connectors relative to each other on the same board by establishing a connector center of guidance.  

Once a center of guidance is established, you may then figure misalignment. Misalignment values influence the acceptance limits for tolerance analysis; even slight misalignment can significantly increase tolerance stack and final assembly pin registration.  

In addition to connectors when evaluating alignment, there are corresponding elements that must also be taken into account, such as:  

• solder pad size and location
• connector placement onto the solder pad prior to reflow
• connector movement during processing
• if the connector self-levels or self-centers
• mating/unmating forces
• if alignment pins or standoffs are used

Samtec does recommend using standoffs as they negate angular and z-axis misalignment effects. However, best practices are to place standoffs as close to the connector system as possible which will localize stress to the vicinity of the connector and decrease unsupported PCB span. It is also recommended to leave standoffs and similar items loose fitted until post connector mating, then mate to final torque specifications. 

Alignment pins are excellent for guiding placement of a connector onto a PCB during hand assembly and for polarizing a connector to the PCB. However, they can contribute to tolerance stack. Therefore, Samtec recommends using a product without alignment pins if machine placing or using in a multi-connector application. If alignment pins are desired for machine placement due to polarization, then oversizing the drill hole on the PCB is recommended.  

Tolerance Analysis Methods 

Tolerance analysis may be achieved from an Arithmetic Worst-Case study or using a Root Sum of Squares study. Which method to use depends on application requirements. For example, some instances require analyzing zero failure targets while with others it is OK to take a less conservative approach where reaching dimensional goals is less difficult and therefore lower cost. 

• Root Sum of Squares (RSS) study:  This is a statistical approach to tolerance stack-up analysis that simulates expected outcomes for the entirety of parts used in a multi-connector application.  

• Arithmetic Worst-Case (AWC) study:  This is a traditional approach when performing a stack-up analysis and is calculated by setting all part tolerances at their limits, resulting in largest or smallest cumulative measurements.  

For tolerance analysis assistance, please reach out to Samtec’s High-Speed Board-to-Board Group at [email protected].  

The Role of a Connector Manufacturer  

All items used to create and manufacture a system have their own tolerances that must be considered in a tolerance stack-up analysis, since it’s the combination of those tolerances that create stacking. Included, but not limited to, are tolerances from both the board house, contract equipment manufacturer, and final assembly process. All of which impact connector function; therefore, Samtec looks to provide general guidance to ensure end system reliability. Helping our customers find the information they need for a successful outcome is all part of Samtec’s Sudden Service® philosophy.  

With that said, as a connector manufacturer, we really are only able to control tolerances of the connectors themselves. Samtec’s goal is to clearly define connector tolerances, max allowable misalignment, footprint and stencil designs, along with processing recommendations. Connector print and footprint information is located at the bottom of each product Series tech spec page (here’s an example: samtec.com?SEAM). For complete processing information, visit samtec.com/processing or contact [email protected]. 

Other helpful links: 

• Samtec’s Multiple Connector Applications Overview document
• SureWare™ Precision Standoffs blog: Need Secure PCB Stacking? You Can Be Sure
• SMPM board-to-board applications blog: SMPM Axial Misalignment? We Can Fix That
• Reliable, High-Speed Ultra Micro PCB Connectors: samtec.com/ultra-micro
• High-Density Array Connectors: samtec.com/arrays
• Connector Supplier article: Meet the Connector: What are Mezzanine Connectors?