The Core Factors Affecting The Solderability Of PCB Pads Are Fully Sorted Out: The Underlying Logic From Materials To The Environment

2026-04-03 16:05

PCB pad solderability is not a fixed attribute, but a dynamic performance that is affected by five dimensions: surface treatment process, manufacturing process, storage environment, soldering conditions, and material matching. The solderability of the same batch of PCBs may vary greatly due to different storage environments and soldering parameters. The attenuation speed and anti-interference ability of weldability are significantly different in different surface treatment processes. This paper delves into the underlying logic, systematically dismantles the core influencing factors of solderability, helps practitioners accurately locate the root causes of poor solderability, and realizes optimal control from the source.

Printed Circuit Board

1. Surface treatment process: the core basis for determining weldability

The surface treatment of the pad is a key barrier to protect the copper surface and improve the solderability, and the wettability, oxidation resistance, storage life, and cost of different processes vary greatly, which is the primary factor affecting the solderability.
 
  1. OSP (Organic Solder Protection Film) Advantages: low cost, excellent wettability, suitable for lead-free processes, high pad flatness; Disadvantages: The protective film is extremely thin (0.2~0.5μm), poor temperature resistance, afraid of scratches, afraid of moisture, and short storage period (3 months ≤ in a dry environment at room temperature). Insufficient film thickness, uneven coating, and excessive baking will lead to the failure of the protective film and rapid oxidation of the pads; Hand sweat and acid-alkali pollution will directly damage the OSP film and cause soldering rejection.
     
     
  2. ENIG (Chemical Nickel Gold) Advantages: long storage life (≥ 12 months), high flatness, suitable for high-frequency and high-speed boards, strong anti-pollution ability; Disadvantages: high cost, easy to black nickel (nickel layer corrosion), gold brittleness defects. The nickel layer thickness < 3 μm is easy to oxidize, the gold layer < 0.05 μm cannot completely cover the nickel layer, and > 0.15 μm can easily lead to brittleness of the IMC layer and cause welding failure.
     
     
  3. Advantages of immersion Ag: good wettability, good heat dissipation, suitable for high-frequency plates, lower cost than ENIG; Disadvantages: poor anti-vulcanization ability, easy to generate silver sulfide in humid environment, resulting in a sharp drop in weldability; The silver layer is too thin and easy to oxidize, and too thick is easy to fall off.
     
     
  4. Immersion Sn Advantages: Excellent wettability, suitable for through-hole soldering, moderate cost; Disadvantages: The tin layer is easy to grow whiskers, easy to oxidize at high temperature and humidity, and the storage period is about 6 months.
     
     
  5. Advantages of tin spraying (HASL): mature process, low cost, stable solderability, and damage resistance; Disadvantages: poor pad flatness, not suitable for high-density patches; The tin surface is easy to oxidize after long-term exposure, and the wettability of lead-free tin spraying is slightly worse than that of lead.
     
     
 
The choice of surface treatment process directly determines the focus of solderability control: OSP board needs to be strictly controlled for storage and transportation, ENIG board needs to control nickel gold thickness and black nickel risk, immersed silver plate needs to prevent sulfur pollution, and tin spraying plate needs to prevent oxidation. In the Jeepai factory, we have developed exclusive testing standards for different surface treatment processes, with 100% film thickness measurement for OSP plates and nickel gold thickness measurement for XRF plates for ENIG plates, eliminating process defects from the source.
 

2. Pollution and defects in the manufacturing process: the direct cause of poor weldability

Residues, damage, and plating defects during PCB manufacturing can directly damage the surface condition of the pads, leading to the failure of solderability.
 
  1. Organic pollution Fingerprint grease, cutting fluid, mold release agent, solder mask ink residue, developer residue, antistatic agent residue, etc., will form a hydrophobic film on the surface of the pad, hindering the wetting of the solder. In particular, poor solder mask holes and ink overflow will cover the edge of the pad, resulting in local non-wetting.
     
     
  2. Oxidation defects After etching, the copper surface is exposed for too long, the copper/electroplating process is abnormal, and the baking temperature is too high, which will lead to oxidation of the copper, nickel, and tin surfaces, forming a dense oxide layer that the solder cannot penetrate.
     
     
  3. Coating defects Pinholes, pockmarks, peeling, leakage plating, and uneven thickness of the coating will lead to local unprotection and rapid oxidation; ENIG black disks, immersed tin whiskers, and tin spraying beads/slag can cause poor solderability.
     
     
  4. Mechanical damage Scratches and bumps during production, cutting, and transportation will damage the surface protective film and coating, expose the metal base, and cause oxidation and welding resistance.
     
     
 
Manufacturing process control is the key to ensuring weldability: the cleaning process needs to be strictly implemented to remove organic residues; optimize electroplating and chemical deposition parameters to ensure that the coating is uniform and complete; strengthen anti-static and dust-proof control to avoid secondary pollution; The finished product is vacuum-packed with built-in desiccant and humidity indicator card.
 

3. Storage and transportation environment: the main driver of weldability attenuation

The solderability of pads dynamically decays with storage time and environmental conditions, and high temperature, high humidity, sulfide, and chloride ions are the three major killers.
 
  1. Temperature and humidity affect Temperature > 30°C and humidity > 60% RH will accelerate metal oxidation and protective film decomposition: OSP plates will fail in 1 month under high temperature and humidity, and the tin spraying plate will be obviously oxidized after 2 weeks, and the silver plate is prone to vulcanization corrosion. Standard storage conditions are: temperature 15~25°C, humidity < 50% RH, vacuum sealed packaging.
     
     
  2. Storage time OSP plates ≤ 3 months, immersed silver/tin plates ≤ 6 months, ENIG/tin sprayed plates ≤ 12 months; Overdue storage must be redone for solderability testing, and it can only be launched after passing the qualification.
     
     
  3. Environmental pollutants Sulfides, chloride ions, acid and alkali gases in the air will corrode the surface of the pads: black silver sulfide is generated by immersed silver plates when encountering sulfide, and ENIG plates are easily corroded by chloride ions to corrode the nickel layer, all of which will lead to complete loss of weldability.
     
     
  4. Improper packaging No vacuum packaging, desiccant failure, and no anti-static bag will lead to direct exposure of the pads, accelerating oxidation and contamination.
     
     
 
In many enterprises, poor soldering is not a quality problem of the PCB itself, but a result of improper storage and transportation. It is recommended to establish a PCB first-in, first-out (FIFO) management system and compulsory re-inspection of overdue boards. The transportation is shockproof, moisture-proof, and anti-static packaging to avoid extreme environmental exposure.
 

4. Welding process parameters: the key variable of on-site weldability

For the same pad, improper soldering parameters will directly manifest as poor solderability, and the core parameters include temperature, time, flux, and preheating.
 
  1. Soldering temperature Too low temperature: the solder is not melted sufficiently, the wettability is poor, and it is easy to have cold soldering and poor tin penetration; Excessive temperature: Accelerates the oxidation of the pad, destroys the OSP film, leads to too thick brittleness of the IMC layer, and the temperature of lead-free solder > 260°C easily damages the pad.
     
     
  2. Dip soldering / reflow time Time is too short: insufficient wetting, poor tin penetration; Too long time: the plating is overly dissolved, the pads are corroded, and the flux fails.
     
     
  3. Flux matching Insufficient flux activity: the oxide layer cannot be removed, resulting in non-wetting; Excessive activity: corroded pads, residual ion pollution; If the flux type does not match the surface treatment, the wetting effect will be greatly reduced.
     
     
  4. Preheating conditions Insufficient preheating: the water vapor of the plate volatilizes to cause bubbles, and the flux is not activated; Overheating: OSP film failure, pad oxidation.
     
     
 
The soldering parameters need to match the PCB surface treatment: the OSP board needs to be moderately preheated and activated flux; ENIG plates avoid high temperature and long soldering; The tin spray plate can be adapted to conventional parameters. The production line needs to establish a standardized welding process window to avoid abnormal weldability caused by parameter drift.
 

5. Material compatibility: The compatibility of the alloy with the coating

After the transition to lead-free, the matching of solder alloy and pad coating has become a new pain point in solderability. SAC305 lead-free solder has a high melting point and poor wettability, which requires higher pad coating: ENIG pads need to ensure that the nickel layer is intact to avoid brittle IMC formed by gold and tin; The sinking soldering pad is compatible with lead-free solder and has stable wettability. OSP pads need to ensure uniform film thickness to help wett the solder.
 
In addition, the moisture absorption of the substrate can also indirectly affect the weldability: high Tg substrates have low hygroscopicity and few bubbles during welding; After the ordinary substrate absorbs moisture, water vapor erupts during welding, destroying the wetting interface and causing pinholes and bubble defects.
 
Weldability is the result of the combined action of materials, processes, environment, and equipment, and a single factor abnormality can cause failure. The control idea should be changed from passive testing to active prevention: optimize and adapt the surface treatment process, strictly control the cleanliness of the manufacturing process, standardize the storage and transportation conditions, match the welding process parameters, and establish a traceability system for the whole process. In the manufacturing of high-end products, it is necessary to combine aging tests (high temperature and high humidity aging, salt spray testing) to verify the long-term reliability of the pads and avoid later service failures.



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