A stencil aperture modification eliminated pogo pin tilt during reflow, maintaining alignment within customer specifications.

Pogo pins are specialized, spring-loaded electrical connectors used to establish reliable temporary or permanent connections between electronic circuits. Comprised of a plunger, barrel and spring, they provide consistent contact force – typically around 1 Newton –ensuring stability against vibration and accommodating thermal expansion in compact devices.

A post-solder reflow inspection revealed SMT pogo pins with angular shifts exceeding the customer’s allowable tolerance of ±0.1° from the required 90° orientation.

Figure 1. SMT pogo pins after reflow soldering, showing angular displacement beyond the customer’s ±0.1° tolerance from the required 90° orientation.

Pogo pins are mounted perpendicular to the PCB surface to ensure maximum structural integrity, reliable electrical contact and even spring compression over thousands of repetitive cycles. Strictly perpendicular mounting prevents the plunger from binding or bending within the barrel, which can lead to premature failure or intermittent connectivity.

They are typically mounted perpendicular to the PCB because their performance and reliability depend on axial compression. When the pin is compressed directly along its centerline, the internal spring operates as intended, providing consistent contact force and minimizing wear. Mounting the pin at an angle can introduce side-loading, causing the plunger to scrape against the barrel, increasing friction and reducing service life.

Maintaining perpendicularity also helps prevent plunger binding. During mating, an angled pin can place uneven stress on the internal spring, increasing the likelihood that the spring will buckle or that the plunger will become stuck in a compressed position. Vertical alignment ensures that spring force is transferred directly to the target pad, providing stable, low-resistance electrical contact and supporting the high cycle counts for which pogo pins are designed.

In applications that use arrays of pogo pins, such as test fixtures, perpendicular mounting is particularly important because even a single misaligned pin can affect overall alignment, resulting in uneven contact pressure and unreliable connections.

Vertical placement also improves manufacturing stability during surface mount assembly, as properly aligned pins are less likely to shift during reflow soldering, reducing the risk of defects such as poor solder joints or electrical shorts.

The root cause was excessive solder paste volume on certain SMT pads. During reflow, the excess solder created enough lift to raise and tilt some of the pogo pins, resulting in angular shifts that exceeded the specified tolerance. Analysis showed that the issue was not related to component placement accuracy, but rather to the amount of solder deposited beneath the pins.

Figure 2. Excess solder paste volume on the SMT pads created lifting forces during reflow, causing pogo pins to tilt and shift out of alignment.

Figure 3. Reducing the stencil aperture to 50% of the pad area limited solder volume and maintained proper pogo pin perpendicularity after reflow without the need for a holding fixture.

To address the problem, the stencil aperture was reduced to 50% of the SMT pad area rather than a full 100% opening.

This adjustment provided sufficient solder volume to meet IPC-A-610 requirements while preventing excess solder from lifting the components during reflow. As a result, the pogo pins maintained proper perpendicular alignment after soldering, eliminating the need for an auxiliary fixture to hold them in position throughout the reflow process.

Akber Roy is CEO and founder of RUSH PCB Inc. in Silicon Valley (Milpitas), California, www.RushPCB.com. Contact the author at sales@rushpcb.com.

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