Analysis of the difference between solder paste mesh and red glue mesh in SMT chip processing

Analysis of technical characteristics and process adaptability of solder paste mesh and red adhesive mesh in SMT chip processing

Introduction: Technical positioning of precision printing templates

In the modern production system of surface mount technology (SMT), solder paste stencil and glue stencil are the core tools for micron-level material distribution, and their performance indicators directly determine the solder joint quality and mechanical reliability of electronic components. According to the latest revision of IPC-7525A, the critical dimensional tolerances of these printed templates must be controlled within ±25 μm, and the hole wall roughness must be ≤ 1.6 μm to meet the assembly requirements of ultra-fine pitch components below 0.3 mm.

As electronic devices evolve to be "light, thin, short, and small", the component density of PCB boards has increased from 50 /cm² in 2000 to 300 /cm² today, and some high-end communication devices have even reached 500 /cm². This trend towards high-density integration presents a dual challenge for printing templates: ensuring accurate dispensing of solder paste/red glue while adapting to the differentiated needs of different components, from 01005 packages to large BGAs. This paper will systematically analyze the technical characteristics and optimization paths of the two templates from three dimensions: material science, process engineering and application practice.

1. Technical system and process control of solder paste mesh

As a key carrier for achieving electrical interconnection, the design and manufacture of solder paste mesh need to balance the three core indicators of conductivity, mechanical strength and process compatibility.

1.1 Comparison of material selection and manufacturing process

At present, the mainstream solder paste mesh material forms a binary technical route of stainless steel and flexible materials: 304/316 stainless steel mesh: thickness range 0.1-0.15mm, with excellent rigidity and corrosion resistance. Among them, 316 stainless steel has a 40% increase in solder paste corrosion resistance compared to 304 due to the addition of 2-3% molybdenum element, and can still maintain dimensional stability (deformation < 5μm) after 50,000 continuous printing. Polyimide (PI) Flexible Mesh: Thickness can be as thin as 0.05mm, suitable for curved printing or flexible PCB processing. By adding glass fiber reinforcement, its tensile strength can reach 150MPa, and the printing accuracy is controlled within ±10μm.

The choice of manufacturing process directly affects the performance of the mesh board:

The electroforming process achieves hole wall verticality of 90°±0.5° through atomic-level deposition on a nickel metal substrate, making it particularly suitable for solder paste printing of 0201 package components (pad size 0.2×0.1mm). Practical data from an Apple foundry shows that the solder paste printing yield of 0201 components has increased from 82% to 97% after using electroformed mesh plates.

1.2 Dynamic optimization strategy of printing parameters

Solder paste printing is a complex process involving fluid mechanics and material science, where precise control of key parameters is particularly important: stainless steel scrapers recommend a contact angle of 60°±5°, which allows the solder paste to achieve optimal shear force (approx. 150Pa) for uniform transfer. For ultra-fine pitch elements, a polyurethane scraper (hardness 80 Shore A) reduces wear on the mesh plate and extends its service life to more than 80,000 cycles. There is a significant interaction between printing speed and squeegee pressure. Experiments show that 20mm/s speed with 30N pressure is suitable for most QFP components. BGA printing needs to be increased to 50mm/s and 45N to ensure that the solder paste is sufficiently filled. For spacing components below 0.5mm, the release distance should be strictly controlled at 0.5-1.0mm, and the release speed should be set at 1/3 of the printing speed (about 15mm/s), which can effectively avoid the phenomenon of "tailing". A 5G base station PCB production line reduced the BGA bridging defect rate from 300ppm to 50ppm by optimizing the release parameters.

1.3 Root cause analysis and solutions of typical defects

The occurrence of solder paste printing defects is often the result of multi-factor coupling, and a systematic diagnostic system needs to be established: Bridging: Except for the large screen opening, the viscosity of the solder paste is low (< 100kPa・). s) and too fast printing speed (>80mm/s) are also important triggers. The solution included using trapezoidal openings with wide top and narrow bottom (upper diameter is 5-10μm larger than lower diameter), and controlling the ambient temperature at 23±2°C to stabilize the solder paste viscosity. Insufficient Paste: Poor release accounts for 65% of the cause and can be solved by spraying a Teflon anti-stick coating (0.5μm thickness) on the bottom of the screen. At the same time, regular (every 2000 prints) ultrasonic cleaning of the mesh plate to remove residual solder paste in the holes can reduce tin deficiency defects by 70%. Tombstoning: Mainly due to the uneven amount of solder paste and the temperature difference between the two ends of the component. This phenomenon is effectively suppressed by ensuring a deviation of < 10% between the left and right pads through 3D SPI (solder paste inspection) and optimizing the reflow temperature curve (90s at 150-180°C in the constant temperature range).

2. Technical characteristics and process adaptation of red rubber mesh

As a core tool for mechanical fixation of components, the design of red glue mesh needs to focus on three key indicators: bond strength, curing characteristics and process compatibility.

2.1 The properties of red rubber materials are related to the design of the mesh board

The performance parameters of epoxy-based red glue directly determine the design strategy of the mesh board: Viscosity characteristics: The viscosity range of 150-350 kcps (25°C) should match the size of the mesh plate cutout. Low-viscosity red glue (150-200kcps) is suitable for printing with a small aperture of 0.3mm, while high-viscosity models (300-350kcps) need to be equipped with holes above 0.5mm to avoid clogging. Curing kinetics: The curing temperature range of 120-150°C requires the mesh material to have good heat resistance. Experimental data show that under the curing condition of 150°C/120s, the thermal deformation of 304 stainless steel mesh is < 2μm, which fully meets the accuracy requirements. Mechanical properties: ≥ shear strength of 6MPa (IPC-TM-650 standard) is the basic requirement, and for vibration environments such as automotive electronics, modified epoxy adhesive (with 10-15% rubber elastomer) needs to be selected, the shear strength can reach more than 8MPa, and the elongation at break is increased to 5%.

2.2 Collaborative design of mesh plate for double-sided placement process

The double-sided mounting process of "red glue + wave soldering" puts forward special requirements for the design of the mesh board: component weight control: the upper limit of 25g corresponds to a specific glue design - for 10g components, 3-4 glue dots with a diameter of 0.8mm need to be designed; 20g components need to be increased to 6-8 with a symmetrical distribution of adhesive dots. Optimize the size of the adhesive dot: The diameter of the adhesive should be 50% larger than the width of the solder end of the component (e.g., 0.4mm solder end corresponds to 0.6mm adhesive end), and the height after curing should be controlled at 0.15-0.25mm, which can ensure sufficient adhesion and avoid solder masking during wave soldering. Mesh board partition design: In mixed PCBs, the red glue mesh needs to adopt a partition thickness design - 0.1mm (fine glue dots) in the IC area and 0.15mm (large glue dots) in the connector area, and differentiated distribution is achieved through a stepped structure.

2.3 Technical solutions for special application scenarios

Red glue mesh shows unique advantages when dealing with complex working conditions: Large heat capacity component fixation: For large components such as BGA and QFP, the red glue mesh is designed into a ring glue dot (width 0.2mm) using the composite process of "peripheral red glue + center solder paste", which can effectively prevent warping during reflow soldering (controlled within 50μm). High-frequency circuit applications: Ceramic-filled red rubber with a dielectric constant of ε<3.5, combined with high-precision red glue mesh (cut-out tolerance of ±5μm), can reduce the transmission loss of 10GHz signal by 0.5dB/cm, meeting the performance requirements of 5G mmWave antennas. Lead-free process compatibility: For the high temperature (260°C) requirements of lead-free solder, silicon-modified epoxy adhesive is used in combination with high-temperature resistant mesh plate (316 stainless steel), which maintains 70% initial bond strength after 10 reflow solders.

3. Process selection framework and technology development trend

3.1 Decision matrix and scenario adaptation model

Multidimensional factors need to be considered for process selection based on product characteristics:

1. Single-panel scenario: Prefer the solderpaste printing process, and the thickness of the screen board is designed according to the component size tier - 0.08mm for the 01005 package and 0.12mm for the SOIC package to ensure accurate matching of the amount of solder paste.

2. Double-sided scenario: Using the mixed scheme of "A-side solder paste/B-side red glue", the red glue mesh should avoid the position of the A-side solder joint, and the distance between the edge of the glue and the pad should be ≥0.2mm to prevent the risk of short circuit during wave soldering.

3. Large-size component scenarios: Implementing the "solder paste + local dispensing" composite process, such as QFPs above 5mm×5mm, can increase the reliability of solder joints by 40% by adding auxiliary adhesive dots at the four corners through a special red glue mesh.

4. Extreme environmental scenarios: High-temperature applications (such as engine control modules) require a combination of high-temperature resistant red rubber (above 260°C) and 316 stainless steel mesh, which retains > 80% bond strength after 1000 hours of aging test.

3.2 Technological innovation and future evolution direction

Printing template technology is developing rapidly in the direction of intelligence and long-term effectiveness: nano-coating technology: deposit diamond-like (DLC) coating (thickness 2-5μm) on the surface of the screen board, the surface energy can be reduced to less than 20mN/m, the solder paste release performance is increased by 50%, and the service life of the screen board is extended to 500,000 times, which is 3 times higher than that of traditional screen boards. Intelligent Sensing Mesh Board: Integrates micro pressure sensor (accuracy ±1kPa) and temperature sensor to monitor the process parameters in the printing process in real time, and uploads them to the cloud analysis platform through the industrial Internet to achieve early warning of defects (accuracy > 90%). Multi-functional Composite Template: Selective plating technology is used to achieve thickness control (0.05-0.2mm) in different areas on the same screen board, meeting the one-time printing needs of complex PCBs (such as containing 01005 components and large connectors at the same time), and increasing production efficiency by 30%.

4. Engineering practice suggestions and quality control system

4.1 Network board management and maintenance specifications

Establishing a full life cycle management system is the key to ensuring printing quality: Usage tracking: each screen board is equipped with RFID tags, automatically recording the number of prints, and when it reaches 20,000 times, accuracy detection is carried out (using 2D image measuring instrument, accuracy ±1μm), and more than 50,000 forced scraps. Periodic calibration system: Printing proficiency verification (CPK≥1.33) is carried out quarterly, and the printing accuracy (deviation < 10%) and consistency (CP≥1.67) of solder paste/red glue are evaluated by designing a standard test plate (including a variety of typical pads). Cleaning process optimization: Adopt "dry wipe + wet cleaning + vacuum" three-stage cleaning process - dry wipe to remove surface residue, ultrasonic cleaning (40kHz) to remove impurities in the hole, vacuum adsorption (-0.08MPa) to ensure dryness, so that the cleanliness of the mesh board is increased to 99.5%.

First article verification and process control

1. New board first article inspection: Implement 3D SPI full inspection, focusing on: solder paste volume: deviation controlled within ±15%, solder paste shape: height/diameter ratio 0.5-0.7, position accuracy: center offset < 25% pad size

2. Process monitoring strategy: 5 PCBs are extracted every hour for random inspection, and X-Ray is used to detect the bottom solder ball of the BGA to ensure that there are no fatal defects such as bridging and less tin, and the process capability index Cpk is maintained above 1.33. Current industry data shows that through the above optimization measures, the defect rate of the solder paste printing process can be stably controlled below 500ppm, and the application of red glue can increase the overall yield of double-sided boards by 15-20%. For companies seeking customized solutions, it is recommended to conduct DFM (Design for Manufacturability) analysis to further improve production efficiency and product reliability through collaborative optimization of network board and PCB design.