Catching the stray solder paste.

Manufacturing defects in surface mount technology (SMT) assembly can prove persistent and challenging to diagnose without a structured troubleshooting approach. When a contract assembler encountered recurring solder balling on a single capacitor, initial efforts to correct the issue through reflow profile adjustments proved ineffective. This case study demonstrates how a systematic troubleshooting framework can effectively identify and resolve assembly defects.

Identifying the Issue: Initial Observations

Solder balling can result from a range of process variables, including stencil printing errors, placement inconsistencies, reflow conditions and contamination. In this case, the defect affected a specific component, occurring inconsistently but repeatedly. Additional issues, including occasional blowholes, further complicated the diagnosis.

The troubleshooting process began with standard diagnostic questions:

• Was the solder paste fresh and properly handled?
• Did defects consistently occur on the same component and location?
• Were boards misprinted, cleaned or rebaked before use?
• Did adjusting the reflow profile impact defect occurrence?

A Systematic Troubleshooting Approach

Despite verifying all standard process parameters, the solder balling issue persisted, prompting further investigation. A four-step evaluation process ensued.

Step 1: Evaluate material and environmental factors. Process engineers confirmed the solder paste was within its shelf life and properly acclimated to room temperature before use. They evaluated boards and components for moisture absorption, but baking trials showed no improvement. Environmental conditions were within acceptable humidity and temperature ranges.

Step 2: Analyze the printing and placement process. The team examined stencil printing quality and ensured no misprinted boards were being reworked improperly. Pick-and-place accuracy was within specification, and placement pressure settings were verified. An unexpected observation emerged, however: the problematic capacitor exhibited “billboarding,” a defect where the component stood on its side rather than lying flat (Figure 1).

Figure 1. Component billboarding, where the part stands on its side.

Step 3: Examine the reflow process. The team optimized a reflow profile, which received approval from QA, but the defect persisted. Given that billboarding is typically a placement-related issue rather than a reflow defect, the team redirected its focus to handling and secondary processes.

Step 4: Investigate secondary and manual handling effects. An inspector revealed the billboarding was an intentional part of the customer’s specification. Operators were manually rotating the component using tweezers before reflow, a step not initially documented as part of the standard process. Further investigation revealed that stray solder paste was being transferred onto the component by dirty tweezers, leading to solder balling during reflow.

Root Cause Identification and Resolution

By isolating manual handling as the contributing factor, the solution was straightforward: provide operators with clean tweezers and reinforce best practices for handling components. A five-board sample run confirmed that the defect was eliminated.

Key Takeaways and Lessons Learned

• Structured troubleshooting prevents misdiagnosis. Rather than assuming the issue stemmed from reflow conditions, a step-by-step approach helped identify an overlooked variable.
• Unexpected process variations matter. Minor undocumented handling steps can introduce defects.
• Cleanliness is critical. Simple tool contamination can lead to persistent defects.

This case highlights the importance of a methodical troubleshooting approach in electronics assembly. By systematically eliminating variables, engineers can pinpoint and correct defects efficiently. Success lies in thorough process evaluation, attention to detail and an open-minded approach to identifying root causes.

Timothy O’Neill is executive director of product management at AIM Solder (aimsolder.com); toneill@aimsolder.com.