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PCBA Incoming Inspection: Comprehensive Process Specifications and Practical Applications

In the quality control system of electronics manufacturing, incoming inspection serves as the first line of defense for product quality, directly determining the stability of subsequent production processes and the reliability of final products. The incoming inspection of PCBA (Printed Circuit Board Assembly) involves multiple materials such as solder paste, stencils, PCBs, various electronic components, and structural parts. Each type of material possesses unique quality attributes and inspection standards. Based on best practices in the electronics manufacturing industry, this article systematically outlines the core technical system for PCBA incoming inspection, detailing inspection items, operational specifications, common issues, and countermeasures for various materials, providing technical support for building a full-process quality control system.

Refined Control Technology for Solder Paste Incoming Inspection

As the core material in SMT processes, the quality stability of pasta solderdirectly affects the performance of the entire printing and soldering process. Solder paste incoming inspection should establish a multi-dimensional evaluation system covering physical characteristics, chemical properties, and process compatibility.

Dynamic Viscosity Testing Technology

Viscosity testing is the most commonly used basic item in solder paste incoming inspection, with results directly reflecting the printing performance of the solder paste. When using a rotational viscometer (e.g., Brookfield DV2T) and testing according to IPC-TM-650 2.4.44 standard, the test environment must be strictly controlled to constant temperature conditions (25±1°C) to avoid viscosity deviations caused by temperature fluctuations (each 1°C temperature change causes approximately 5–8% viscosity deviation).

In practice, common stability issues often stem from improper pre-test handling:

Insufficient thawing time: Solder paste taken from refrigeration must rest at room temperature (23±2°C) for at least 4 hours; inadequate thawing will result in viscosity test values being 15–20% higher;

Inconsistent stirring parameters: When using an automatic stirrer, set the rotation speed to 300 r/min for 2 minutes; manual stirring cannot guarantee uniformity, potentially causing viscosity deviations of up to 70 Pa·s within the same batch (e.g., differences between 240 Pa·s and 170 Pa·s);

Improper testing timing: After stirring, allow 10 minutes of rest before testing; immediate testing will yield lower results due to incomplete shear force release.

Standardizing the testing process can control viscosity test deviations within ±15 Pa·s for the same batch. If test values exceed the specification range of 150–250 Pa·s, comprehensive judgment should be made based on the characteristic curve of the solder paste type, and if necessary, the supplier should provide the factory inspection report of the same batch for comparison.

Accurate Determination of Flux Content

Minor changes in flux content (typically 8–12%) in solder paste significantly affect solder joint formation quality. When using the gravimetric method, precisely weigh 5.000 g of solderpaste (accuracy 0.001 g), heat on a 260°C constant temperature hot plate until completely melted (about 5 minutes), then use isopropanol for ultrasonic cleaning (30 kHz, 5 minutes) to remove flux residue, followed by drying at 80°C for 30 minutes and weighing the metal residue mass to calculate flux content (Flux Content = (Initial Mass - Metal Mass) / Initial Mass × 100%). Key control points of this test include:

Cleaning thoroughness: Solvent must be changed 3 times to ensure no residue, otherwise test values will be low;

Drying temperature control: Exceeding 100°C may cause solder powder oxidation and weight gain, introducing positive deviation;

Number of parallel samples: Conduct at least 3 sets of parallel tests, take the average as the final result, with relative deviation of individual samples ≤1%.

When widespread insufficient solder or excessive flux residue occurs on the production line, this test can serve as a key method for tracing solder paste quality. Practical cases show that when flux content decreases from 10% to 9.5%, the solder volume for 0402 components decreases by about 8%, with welding strength correspondingly decreasing by 12%.

Copper Mirror Corrosion Test Method

The copper mirror test evaluates the corrosiveness of solder paste flux, operated according to JIS Z 3197 standard method: Evaporate a 500 nm thick pure copper film (with mirror effect) on a glass slide, print 0.1 g of solder paste and cover with a glass slide, place in a 125°C oven for 24 hours, then observe whether corrosion spots or discoloration appear on the copper film. Note for result determination:

Grade classification: Level 0 (no corrosion), Level 1 (slight discoloration), Level 2 (local corrosion), Level 3 (severe corrosion); electronic-grade solder paste should achieve Level 0–1;

Environmental control: Tests should be conducted in a clean room (Class 1000) to avoid interference from airborne pollutants like sulfur and chlorine;

Reference samples: Each test must include a blank copper mirror without solder paste contact as a reference.

When electrochemical migration of solder joints occurs on the production line (e.g., dendrite growth in humid environments), the copper mirror test can quickly determine whether flux corrosiveness exceeds standards. A communication equipment company discovered through this test that a certain batch of solder paste reached corrosion Level 2; after replacement, migration defect rate decreased from 0.5% to 0.02%.

Quantitative Analysis of Solder Spreadability

Spreadability testing is a comprehensive indicator for evaluating the soldering activity of solder paste. During operation, print solder paste on a standard copper sheet (10 mm×10 mm×0.3 mm), heat according to the actual reflow process curve, then measure the ratio of solder spread area to initial printed area (Spreadability = Spread Area / Initial Area × 100%). Test results require multi-factor analysis:

Qualification standard: Spreadability should be ≥80%, with continuous spread edges without sawtooth patterns;

Abnormal judgment: Spreadability <70% may stem from insufficient flux activity or solder powder oxidation;

Comparative application: Simultaneously testing spreadability on PCB pads and component pins can distinguish responsible parties for soldering defects (PCB, components, or solder paste).

When addressing QFP pin solder climbing issues, this test can effectively identify whether it is due to insufficient solder paste activity (65% spreadability) or PCB pad oxidation (58% spreadability), providing basis for targeted improvements.

Dual-Dimensional Evaluation of Slump Characteristics

Solder paste slump characteristics require comprehensive evaluation through both cold slump and hot slump tests:

Cold slump test: Print 0.5 mm wide solder paste lines on PCB, measure line width change after 1 hour at room temperature; qualification standard is line width increase ≤20%;

Hot slump test: Measure the same printed pattern after preheating at 150°C for 30 seconds; line width increase should be ≤30%.

For 0.4 mm pitch CSP devices, cold slump exceeding 10% may cause post-soldering bridging. Practical data shows that solder paste with rosin-based flux has 15% lower hot slump than synthetic resin types, making it more suitable for fine-pitch assembly.

Solder Ball Generation Control Detection

The solder ball test is the most cost-effective method for screening solder paste quality. Standard operation is as follows:

Print f6.5 mm circular solder paste (mass 0.2 g±0.01 g) on ceramic substrate;

Heat according to actual reflow curve, count number of solder balls with diameter ≥0.1 mm after cooling;

Qualification standard: Number of solder balls with diameter ≥0.3 mm is 0, number of 0.1–0.3 mm solder balls ≤3.

A simplified scheme can be performed on large-area solder mask regions of PCB, but note:

Solder mask material influence: Matte solder mask is more prone to solder ball generation than glossy solder mask;

Heating method differences: Hot air reflow typically produces 20–30% fewer solder balls than infrared heating;

Counting method: Use 20x microscope observation to avoid missing tiny solder balls.

This test effectively reflects solder powder oxygen content (>0.15% significantly increases solder balls) and flux inhibition capability. In one case, the solder ball test detected abnormalities in a solder paste batch in advance, avoiding 2% bridging defects after going online.

Solder Powder Microscopic Characteristic Analysis

The particle size and shape of solder powder directly affect printing accuracy, analyzable using metallographic microscope (500x) or laser particle size analyzer:

Particle size distribution: Type 3 solder powder (25–45 mm) must meet D10≥25 mm, D50=35±5 mm, D90≤45 mm;

Shape factor: Sphericity should be ≥0.85 (ideal sphere is 1.0), proportion of irregular particles ≤5%;

Agglomeration state: Number of agglomerates with ≥3 particles adhering in a single field of view should be ≤2.

When addressing 0201 component printing insufficient solder issues, a batch of solder powder was found to have actual D50 of 48 mm (standard ≤45 mm), causing a 30% increase in stencil aperture clogging rate; the problem was resolved after switching to qualified solder powder.

Precision Measurement Technology for Stencil Incoming Inspection

As the key tool for solder paste printing, stencil quality directly determines the accuracy of printed patterns. Stencil incoming inspection should establish a multi-dimensional detection system covering geometric accuracy, surface quality, and mechanical properties.

Comprehensive Aperture Accuracy Detection

Use stencil inspection machine (e.g., Vi Technology V510) for full-size detection, requiring: Position deviation: Deviation between aperture center and design coordinates ≤±0.015 mm;

Dimensional accuracy: Deviation of aperture length/width from design value ≤±0.01 mm;

Aperture wall perpendicularity: ≥85° (angle with stencil surface), avoiding "hourglass" shaped solder paste cross-section during printing.

Note before detection:

Baseline calibration: Use standard calibration plate (accuracy ±0.001 mm) to calibrate equipment daily;