What are the reasons for PCB pads not being tinned and vias not working?

In electronics manufacturing and PCB (Printed Circuit Board) production process, the pads do not tin and through-hole is a common process defects, may be caused by design, materials, process or environmental factors. This article will systematically sort out the root causes of these two types of problems from a technical point of view and provide targeted solutions.

I. the five core reasons why the pads do not go on the tin

Surface contamination and oxidation

Oil, fingerprints or dust: hand contact or environmental dust adhesion will form an insulating layer, preventing solder infiltration.

Oxide layer: Copper foil exposed to the air is prone to generate copper oxide, which needs to be protected by chemical immersion gold (ENIG), OSP and other processes.

Solution: Strict clean room management, plasma cleaning or chemical cleaning to remove contaminants before soldering.

Solder pad design defects

Too small size: the width of the pad is not wide enough to form a reliable connection.

Improper spacing: The spacing between adjacent pads is too close to cause bridging, too far away will lead to false soldering.

Unreasonable shape: Shaped pads (e.g., sharp corners) may cause stress concentration, affecting solder quality.

Solution: Follow IPC design specifications, optimise pad size and layout, and add teardrop design if necessary.

Material compatibility issues

Surface treatment process mismatch: e.g. immersion gold layer is too thin, OSP film is outdated or tin spray layer roughness is insufficient.

Solder composition conflict: Differences in compatibility between lead-free solder and lead-containing components may result in reduced wettability.

Solution: Harmonise surface treatment standards and select substrates and coatings that match the solder.

Uncontrolled soldering process parameters

Insufficient temperature: Preheating or soldering temperature is too low and solder is not fully melted.

Inadequate time: Inadequate wetting due to too short a soldering time and possible damage to components if too long.

Flux failure: Insufficient flux activity or uneven distribution, unable to effectively remove oxides.

Solution: Optimise the reflow profile through DOE experiments and calibrate the equipment parameters regularly.

Component Pin Problems

Pin oxidation or plating defects: e.g. nickel plating is too thick or gold plating is too thin.

Pin deformation: bending or tilting leads to poor contact with the pad.

Solution: Strengthen the incoming material inspection, using high-precision mounter to ensure the positioning accuracy of components.

II. the four key triggers of the over-hole does not work

Drilling process defects

Hole diameter deviation: drilling size exceeds the design tolerance, resulting in subsequent plating difficulties.

Roughness of hole wall: burrs caused by wear and tear of drilling bits or improper parameters, hindering the penetration of plating solution.

Solution: Regular replacement of drill bits, optimisation of drilling speed and feed rate, use of CCD to detect hole diameter.

Plating layer quality problems

Insufficient copper layer thickness: Insufficient plating time or current density, resulting in uneven copper layer coverage in the hole.

Hole depression: uneven distribution of plating solution leads to "dog bone effect", the copper thickness of hole is lower than the centre.

Solution: Adopt pulse plating technology and increase the conductive design of the backplate to improve current distribution.

Plug holes and resin contamination

Green oil hole plugging residue: incompletely cured resin clogging the holes, affecting the flow of plating solution.

Chemical residue: Incomplete cleaning leads to corrosion of the hole wall by acidic or alkaline substances.

Solution: Optimise the hole plugging process parameters and add ultrasonic cleaning.

Mechanical stress damage

Sub-board stress: V-Cut or stamping sub-panel cracks, cutting off the conductive path.

Bending deformation: PCB is bent by external forces, resulting in over-hole fracture.

Solution: Adopt laser depaneling technology, increase the thickness of PCB stacking or design stress relief groove.

III. Comprehensive solutions and preventive measures

Establish DFM (Design for Manufacturing) process: simulate the welding and plating process in the design phase to avoid risks in advance.

Enhance process control: Monitor key parameters (e.g. temperature, plating current) through SPC statistical process control.

Introduction of AOI and X-Ray inspection: automated optical inspection of pad quality and X-Ray to visualise the internal structure of vias.

Supplier co-management: establish quality agreements with PCB manufacturers to clarify standards for surface treatment, plating thickness, etc. Conclusion: The problem of non-tinning of pads and non-compliance of holes needs to be solved through the whole chain of control of "design-materials-process-inspection". Enterprises should be combined with their own product characteristics, the development of targeted failure analysis (FA) process, and continuously optimise the manufacturing process to improve PCB yield and product reliability.