PCB Pad Solderability Failure Analysis: From Defect Location To Root Cause Cracking

2026-04-03 16:20

In PCBA production, poor pad solderability is the primary cause of soldering defects, which are commonly manifested as non-wetting, semi-wetting, tin shrinkage, poor tin penetration, pinhole bubbles, virtual soldering, cold soldering, etc. Failure analysis is not a simple replacement of materials, but a standardized process of "on-site observation→ sample preparation→ instrument detection→ mechanism derivation →process verification" to accurately locate the root cause of defects and avoid recurrence.

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The core logic of failure analysis is reverse traceability: starting from the appearance of soldering defects, eliminating interfering factors such as soldering process, solder, flux, etc., locking in the material, coating, cleanliness, and oxidation state of the PCB pad itself, and finally giving a practical improvement plan. The analysis process should follow the principle of "field first laboratory, qualitative first and then quantitative, simple first and then complex" to efficiently save time and cost.
 

Step 1: On-site defect collection and preliminary judgment

First, collect on-site defect samples and record complete production information: PCB surface treatment type, production batch, storage time/environment, soldering parameters (temperature/time/flux), defect location, defect ratio, and defect rate trend. Observing the defect morphology through 10x magnification / metallographic microscope, preliminary classification:
 

  1. No wetting: the solder is not spread at all, the metal of the pad is exposed, and there is no adhesion → There is a high probability that the pad is severely oxidized, organic contaminated, and the plating fails completely;

  2. Semi-wetting: solder spreads first and then retracts, partially exposed → local defects in the coating, light oxidation, and insufficient flux activity;

  3. Shrinkage: The solder shrinks into a spherical shape, and only dots are attached to → the surface energy is extremely low, heavy pollution, and the OSP film is completely destroyed.

  4. Poor tin penetration: the through-hole hole wall is not wetted → hole wall pollution, coating leakage, insufficient preheating, and the dip soldering time is too short;

  5. Pinhole bubbles: cavities in the solder layer → the board absorbs moisture, flux water vapor, and pad oxide layer decomposition;

  6. Black discs are accompanied by non-wetting: ENIG pads are blackened → typical nickel layer corrosion failure.

 
Preliminary judgment needs to exclude process factors: the same batch of PCB, the defect disappears after replacing the soldering parameters/flux, which is a process problem; If multiple devices and multiple debugging are still bad, the problem with the PCB pad itself is locked. At the same time, the solderability test results of the same batch of unsoldered PCBs are compared, and if the incoming test is unqualified, the defects of the incoming PCB can be directly determined.
 

Step 2: Standardized solderability retest verification

The defective samples and the intact samples of the same batch were retested for solderability, and the combination of edge dipping and welding method + wetting balance method was used to ensure objective results. The test conditions strictly follow IPC J-STD-003, unify the solder, flux, temperature, time, and eliminate human interference.
 
Retest objectives: 1. confirm that defects can be reproduced and exclude accidental factors; 2. Quantify the wetting force, wetting angle, and spread area, and compare the differences; 3. Verify the degree of attenuation of weldability after aging. If the retest results are consistent with the site, they can enter the laboratory for in-depth testing; If the retest is qualified, it means that the on-site process parameters are drifting or improper operation.
 

Step 3: Laboratory instrument in-depth testing

Instrumental testing is at the heart of failure analysis, accurately locating the root cause through microscopic topography, composition analysis, coating thickness, and surface cleanliness testing. Commonly used equipment includes:
 
  1. Metallography Microscope / SEM Scanning Electron Microscope Observe the microscopic morphology of the pad: oxide layer thickness, plating pinholes, peeling, black nickel, whiskers, organic residues, and IMC layer morphology. The SEM can be magnified up to thousands of times to clearly identify nanoscale defects such as corroded holes in the nickel layer of ENIG black disks, OSP film breakage cracks.
     
     
  2. EDS energy spectroscopy detects the elemental composition of the pad surface: if there is a high content of O (oxygen), it indicates severe oxidation; high content of C (carbon), which proves organic pollution; High content of S (sulfur)/Cl (chlorine), proving corrosion of sulfide / chloride ions; ENIG pads have too low Au content and abnormal Ni content, proving that the plating is ineffective.
     
     
  3. XRF Coating Thickness Gauge Non-destructive measurement of coating thickness: OSP film thickness of 0.2~0.5μm is qualified, ENIG nickel layer is 3~5μm, gold layer 0.05~0.15μm is qualified, and the thickness of immersion tin/silver layer is up to standard. Insufficient or severely uneven thickness directly leads to the failure of weldability.
     
     
  4. Surface cleanliness test (ion contamination test) detects ion residues on the surface of the pad: chloride ions, sodium ions, potassium ions, etc. exceed the standard, which will damage the wetting interface, cause corrosion and soldering rejection. Industry standards require ionic contamination < 1.56 μg/cm² (NaCl equivalent).
     
     
  5. Wetting Balance Tester Quantitative Analysis of Wetting Force - Time Curve: Compared with qualified samples, defective samples usually show negative wetting force, too long wetting time, 90% of F5
     
     
 

Step 4: Failure mechanism derivation and root cause localization

Combined with the appearance observation, retest results and instrument data, the failure mechanism is derived and the root cause is identified:
 

Typical failure case 1: OSP board does not wet a large area

 
Phenomenon: The whole plate pad is not wetted, and the retest is still bad; EDS detects high O content and OSP film thickness < 0.15 μm. Root cause: abnormal OSP coating process, insufficient film thickness; storage expiration + high temperature and high humidity, the protective film is completely decomposed; The film layer is damaged by scratching during transportation.
 
 

Typical Failure Case 2: ENIG pad semi-wetted + black disk

 
Phenomenon: The pad is locally blackened, and the semi-wetting ratio is high; The SEM showed corroded holes in the nickel layer, and EDS detected abnormal Ni/O ratios. Root cause: ENIG process nickel tank pollution, pH loss of control, resulting in nickel layer corrosion; The gold layer is too thin to protect the nickel layer and is stored for long-term oxidation.
 
 

Typical failure case 3: Vulcanization of immersed silver plate refuses welding

 
Phenomenon: The pad is black and brittle, and it is not wetted at all; EDS detects high S levels. Root cause: The storage environment contains sulfide gas, and the silver layer forms silver sulfide, which loses weldability. The packaging seal failed, and the moisture-proof and anti-static bag was not used.
 
 

Typical failure case 4: Poor tin spraying plate penetration

 
Phenomenon: The hole wall of the through-hole pad is not wet, and the surface is wetted normally; SEM shows organic residues in the pore wall. Root cause: incomplete cleaning of the hole wall during the PCB manufacturing process, developer / solder mask residue; The wave soldering is not preheated enough, and the flux cannot penetrate the residual layer.
 
 

Typical failure case 5: Batch tin reduction

 
Phenomenon: The solder is all shrunk into a ball shape and there is no spreading; Surface cleanliness test ions exceeded the standard. Root causes: organic pollution in the manufacturing process (grease, release agent); Employees touch the pads with their bare hands, and fingerprints remain; The cleaning process fails.
 
 
Through mechanism derivation, PCB manufacturing defects, storage and transportation defects, and on-site process defects can be clearly distinguished, avoiding blind accountability and repeated trial and error.
 

Step 5: Process verification and improvement plan implementation

Develop improvement plans for root causes and verify the effectiveness through low-batch trial production:

  1. Coating defect improvement: OSP coating parameters are adjusted to ensure uniform film thickness; Optimize ENIG nickel gold process to eliminate black disks; strengthen electroplating control to avoid leakage and peeling;

  2. Pollution control improvement: upgrade the cleaning process to reduce ionic residue; Perform anti-static and dust-free operation, and do not touch the pad with bare hands; optimize the solder mask process to prevent ink overflow;

  3. Storage and transportation improvement: strict vacuum packaging, increase desiccant and humidity card; Perform FIFO management and control storage cycles; Improve storage temperature and humidity to avoid sulfide/chloride ion pollution;

  4. Process matching improvements: optimized welding temperature/time/preheating to match surface treatment types; Choose the adapted flux to improve activity and compatibility;

  5. Control and upgrade: increase the proportion of factory weldability sampling inspection, increase aging testing for key products; establish a re-inspection system for incoming materials, and compulsory testing of overdue plates.

 
The ultimate goal of failure analysis is to prevent recurrence, not a one-time solution. Enterprises should establish a standardized failure analysis process, train professional analysts, and combine solderability testing and instrument testing to form a closed loop of "defect collection, analysis and positioning, improvement verification, and control upgrade".
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