Detailed explanation of the solder paste process in the phenomenon of non-wetting

Non-wetting refers to the soldering process, the solder fails to adequately cover the surface of the substrate pads or device pins, resulting in the formation of a large contact angle between the solder and the base metal (usually > 90 °), and did not form an effective metallurgical bonding phenomenon.The typical characteristics of the solder joint surface showing the base metal colour (such as copper, nickel), the solder is only partially attached or a ball-shaped aggregation (shown in Figure 1-1).This problem directly affects the mechanical strength, thermal cycle reliability and long-term stability of the solder joint.

Figure 1-1 Non-wetting

1. the formation mechanism and core causes

The essence of non-wetting is that the interface reaction between the solder and the base metal is blocked, the specific causes can be divided into the following four categories:

Abnormal state of the substrate metal surface

Oxidation and contamination: pad or pin surface oxide layer (such as CuO, NiO), organic pollutants (grease, flux residues) or intermetallic compounds (such as Au-Sn brittle layer).

Plating defects:

ENIG (Electroless Nickel Immersion Gold Plating): Nickel layer porosity, excessive phosphorus content (>8wt%) or too thin a gold layer (<0.05μm).

OSP (Organic Solder Sheet): The film layer is too thick (>0.5μm) or incomplete thermal decomposition, hindering solder infiltration.

HASL (hot air levelling): micro-cracks appear at the edge of the pad due to thermal stress, exposing the internal copper oxide layer.

Failure of solder alloy and flux

Solder impurities: aluminium (Al), cadmium (Cd), arsenic (As) and other impurities such as elemental segregation, or solder powder oxidation (oxide content > 0.2wt%).

Insufficient flux activity:

Active agent (such as organic acids, halides) concentration is too low or high temperature decomposition failure.

Too long exposure time after solder paste printing (> 4h), flux volatilisation or moisture absorption.

Reflow soldering process parameter deviation

Abnormal temperature profile:

Preheating zone temperature is insufficient (<120℃) or time <60s, flux is not fully activated.

Peak temperature is too high (＞245 ℃) or time above liquid (TAL) ＞60s, accelerated metal oxidation.

Improper atmosphere control:

Oxygen content ＞1000ppm (not using nitrogen protection), resulting in high temperature oxidation of the pad.

Soldering chamber humidity ＞10%RH, water vapour reacts with flux to produce air holes.

Design and process suitability issues

Fine-pitch devices: template opening size < pad size (such as 0.1mm gap), resulting in insufficient solder paste printing, edge wetting.

Mixed assembly process: BGA and plug-in devices common furnace welding, thermal quality differences lead to local temperature unevenness.

the impact and risk assessment

The impact of non-wetting on solder joint reliability needs to be assessed in conjunction with the following dimensions:First, the formation mechanism and core causes

The essence of non-wetting is that the interface reaction between the solder and the base metal is blocked, the specific causes can be divided into the following four categories:

Abnormal state of the substrate metal surface

Oxidation and contamination: pad or pin surface oxide layer (such as CuO, NiO), organic pollutants (grease, flux residues) or intermetallic compounds (such as Au-Sn brittle layer).

Plating defects:

ENIG (Electroless Nickel Immersion Gold Plating): Nickel layer porosity, excessive phosphorus content (>8wt%) or too thin a gold layer (<0.05μm).

OSP (Organic Solder Sheet): The film layer is too thick (>0.5μm) or incomplete thermal decomposition, hindering solder infiltration.

HASL (hot air levelling): micro-cracks appear at the edge of the pad due to thermal stress, exposing the internal copper oxide layer.

Failure of solder alloy and flux

Solder impurities: aluminium (Al), cadmium (Cd), arsenic (As) and other impurities such as elemental segregation, or solder powder oxidation (oxide content > 0.2wt%).

Insufficient flux activity:

Active agent (such as organic acids, halides) concentration is too low or high temperature decomposition failure.

Too long exposure time after solder paste printing (> 4h), flux volatilisation or moisture absorption.

Reflow soldering process parameter deviation

Abnormal temperature profile:

Preheating zone temperature is insufficient (<120℃) or time <60s, flux is not fully activated.

Peak temperature is too high (＞245 ℃) or time above liquid (TAL) ＞60s, accelerated metal oxidation.

Improper atmosphere control:

Oxygen content ＞1000ppm (not using nitrogen protection), resulting in high temperature oxidation of the pad.

Soldering chamber humidity ＞10%RH, water vapour reacts with flux to produce air holes.

Design and process suitability issues

Fine-pitch devices: template opening size < pad size (such as 0.1mm gap), resulting in insufficient solder paste printing, edge wetting.

Mixed assembly process: BGA and plug-in devices common furnace welding, thermal quality differences lead to local temperature unevenness.

2. the impact and risk assessment

The impact of non-wetting on solder joint reliability needs to be assessed in conjunction with the following dimensions:

3.  System level solutions

Material Optimisation

Surface treatment of solder pads: give priority to ENEPIG (Ni-Pd-Au) or immersion silver (ImAg), avoid pure tin or OSP process.

Solder selection: use SAC305 (Sn96.5Ag3.0Cu0.5) alloy, control the oxygen content of solder powder <0.1wt%.

Flux Upgrade: Use high activity no-clean flux (such as RA type) to enhance the wetting ability.

Process control

Temperature profile optimisation:

Preheating zone: 120~150℃, time 90~120s.

Reflow zone: peak temperature 235~245℃, TAL＜45s.

Nitrogen protection: Maintain oxygen concentration <500ppm during welding process.

Design and Process Adaptation

Stencil design: fine-pitch devices using a step stencil (StepStencil), the opening size ≥ pad size 0.05mm.

Printing parameters: Squeegee pressure 0.1~0.2MPa, printing speed 20~50mm/s, to ensure that the fill rate of solder paste>90%.

Process control

Pre-solder cleaning: Use plasma cleaner to remove organic matter on the pad surface.

AOI Inspection: Detect the volume of solder joints by 3D laser, combined with the wetting angle algorithm to screen defective products.

Typical Case Study

Case 1: BGA edge solder ball not wetting

Phenomenon: X-ray inspection found that the BGA corner solder ball void rate> 30%, wetting angle> 110 °.

Root cause: the template opening is 0.15mm smaller than the pad, resulting in insufficient solder paste printing; reflow soldering peak temperature of 255 ℃ (10 ℃ beyond the specifications).

Countermeasures: Modify the size of the template opening, reduce the peak temperature to 240 ℃, the void rate down to 5%.

Case 2: poor wetting of OSP pads

Phenomenon: pin solder joints are "shrinking tin", wetting angle > 90 °.

Root cause: OSP film thickness of 0.8μm (exceeding the standard 0.3μm), flux activity is insufficient.

Countermeasures: replace the OSP process with a low thickness (0.3μm), upgrade the flux activity level, and reduce the wetting angle to 45°.

Through the synergistic optimisation of material, process and design, the non-wetting problem can be effectively controlled, thus improving the long-term reliability of solder joints.