Through-Hole Component Solder Joints

In the electronICs manufacturing industry, ensuring the reliability of through-hole component (THT) soldering is paramount. Solder joints must not only make electrical connections but also withstand mechanical stress, vibration, and thermal cycling. The Pull Strength Test is a core method for quantitatively evaluating the mechanical strength of solder joints. Common standards like "≥5N" are a concrete manifestation of this reliability.

I. Core Purpose of the Pull Strength Test

The pull strength test is not designed to SIMulate real-world stresses (which are often shear or bending stresses) but serves as a destructive testing method for process control and quality assurance. Its main purposes are:

1. Verify Soldering Process: Confirm that parameters like soldering temperature profile, flux activity, and solder alloy are correctly set to form a metallurgically sound joint.

2. Evaluate Solder Joint Strength: Quantify the mechanical strength of the joint itself, ensuring it can withstand subsequent assembly (e.g., SCRew fastening), transportation vibration, and accidental stress during use.

3. Identify Potential Defects: Reveal defects difficult to see with the naked eye, such as cold solder joints, disturbed joints, excessive voids, or poor pin/pad wetting. These defects significantly reduce pull strength.

II. Origin and Basis of the Standard Value (Why ≥5N?)

"≥5N" is a common minimum acceptance criterion, but it is not absolute. Its value is derived from the following considerations:

1. Industry Standards (IPC Standards):

   • The authoritative standards IPC-J-STD-001 (Requirements for Soldered Electrical and Electronic Assemblies) and IPC-A-610 (Acceptability of Electronic Assemblies) are the primary references.

   • These standards typically do not specify a single, fixed force value (like 5N) but rather a principle: the solder joint must exhibit adequate strength. The failure mode (where the break occurs) is more informative than the force value alone.

   • Acceptable Result: The failure occurs in the component lead itself (the lead breaks) rather than at the solder joint or pad interface. This proves the joint's strength is greater than the lead's strength.

   • Unacceptable Result: The solder joint tears away from the PCB pad (pad lift-off) or the joint itself fractures. This indicates a process issue where the joint strength is less than the lead strength.

2. Component Specifications and Safety Factors:

   • 5N is a generic requirement often applied to common axial/radial components (e.g., resistors, capacitors, diodes). This value is based on extensive experimentation and statistics, providing a significant safety margin far exceeding the maximum stress these components would encounter in normal environments.

   • For larger, heavier components (e.g., electrolytic capacitors, transformers, connectors), the required standard value is significantly higher (e.g., ≥20N or more). This value is calculated based on the component's weight, forces in vibrational environments, etc., and multiplied by a safety factor (usually 1.5 to 2 times).

III. Test Method and Procedure

1. Test Equipment: Dedicated push-pull force tester (Digital Force Gauge). This equipment applies force at a constant speed and precisely records the force curve and peak force.

2. Test Fixtures:

   • Specially designed for the component shape. For example, use a hook fixture for axial resistors, a collet fixture for radial capacitors, and a custom fixture to grip the body of a connector.

   • Core Principle: The fixture must ensure the force is applied vertically and uniformly to the component body. The line of force must be perpendicular to the PCB to ensure pure tensile load, not a bending moment.

3. Test Steps:
   a. Sampling: Cut a sample containing the component under test from a production board or a dedicated test coupon.
   b. Secure: Firmly clamp the sample PCB onto the test platform's base fixture.
   c. Attach Fixture: Select the appropriate fixture, attach it to the force gauge sensor, and then connect it correctly to the component body.
   d. Set Parameters: Set the test speed (typically 5-25 mm/min). Speed that is too fast or too slow can affect results.
   e. Perform Test: Start the equipment. Apply tension at a constant speed until the solder joint or lead fails.
   f. Record Results: Record the peak pull force (in Newtons, N) and the failure mode (lead break, pad lift-off, solder joint fracture, etc.).

4. Result Judgment:

   • Pass: Pull force ≥ specified standard value (e.g., 5N) AND the failure mode is lead break.

   • Fail:

     • Pull force < specified standard value.

     • Pull force is达标 (meets the value), but the failure mode is pad lift-off or solder joint interface separation. This is a "false pass," indicating a critical issue with PCB fabrication (pad adhesion) or the soldering process.

IV. Key Factors Affecting Pull Strength

1. Solder Quality: Good wettability, proper solder fillet, and adequate solder volume are the foundation of a strong joint.

2. PCB Design and Quality: Pad size, copper thickness, and the adhesion between the PCB laminate and copper (peel strength) are critical. If the copper separates from the substrate, the PCB itself is faulty.

3. Lead Material and Solderability: The plating on the lead surface (e.g., tin, gold) and its oxidation level directly affect solderability.

4. Solder Alloy: The mechanical strength varies slightly between different alloys (e.g., SAC305, Sn63/Pb37).

V. Common Misconceptions and Mitigation

1. Misconception 1: Qualifying a batch based on 1 sample test

• Consequence: Sample variability may cause misjudgment (e.g., one joint meets standards due to excess solder, while others do not).
• Mitigation: Per IPC standards, test 5-10 samples per component type. A batch is qualified only if all samples meet the standard (no values below the threshold).

2. Misconception 2: Excessively fast pulling speed (>20mm/min)

• Consequence: Instantaneous impact inflates test values (e.g., actual 5N pull force may show 7N), masking true strength deficiencies.
• Mitigation: Strictly control the pulling speed to 5-10mm/min, simulating slow stress (e.g., plugging/unplugging, vibration) experienced in real-world use.

3. Misconception 3: Ignoring failure mode analysis

• Consequence: Focusing only on pull force values without identifying root causes (e.g., pad peeling may stem from poor PCB copper adhesion, not soldering issues).
• Mitigation: Analyze failure modes after testing:
  • "Lead breakage": Solder joint is qualified; failure originates from lead material.
  • "Solder joint separation": Optimize soldering temperature or flux.
  • "Pad peeling": Improve PCB copper adhesion (e.g., use higher-quality PCB substrates).

Conclusion:
"≥5N" is a quantitative threshold for the mechanical reliability of through-hole solder joints. The core value of the pull test lies in evaluating both the force value and the failure mode, providing a comprehensive diagnosis for potential defects from PCB fabrication to the soldering process. It is an indispensable quality tool for ensuring the robustness and longevity of electronic products throughout their lifecycle.