What are the methods of BGA ball planting?

Panoramic analysis of BGA pelleting technology: method comparison and process optimization

In the manufacturing process of ball grid array packaging (BGA), the ball planting process is like installing a "communication tentacle" on the chip, which directly determines the reliability of the electrical connection between the package and the PCB. This precision of deploying micron-sized solder balls on a pad array is both a core part of BGA packaging and a key bottleneck in production yield. With the evolution of electronic devices to high-density and miniaturization, the BGAsolder ball spacing has been reduced from 1.27mm in the early days to 0.3mm or even smaller, and the requirements for ball implantation accuracy have reached ±25μm. This paper systematically sorts out the process characteristics of the four mainstream ball transplanting technologies, deeply analyzes their technical principles, parameter control points and applicable scenarios, and provides accurate process selection guidelines for different production needs.

1. BGA ball planting technology basics

The BGA package is electrically interconnected through a matrix of solder balls at the bottom, and the quality of the pellets directly affects three key metrics: the mechanical strength of the solder joints (the shear force requirement of 5kgf/ball of the JEDEC JESD22-B117A standard must be met), signal transmission integrity (parasitic inductance is controlled below 5nH), and thermal conductivity (thermal resistance coefficient ≤1.5°C/W). To understand the underlying logic of ball planting technology, we need to start from the characteristics of solder balls and the essence of the process.

1.1 Material characteristics of solder balls

The mainstream BGA solder balls feature SAC305 lead-free alloy (tin - 3% silver - 0.5% copper), and its key physical parameters include:

Melting Point: 217°C (34°C higher than traditional tin-lead alloys), Diameter Tolerance: ±0.02mm (J-STD-006 Class 3 compliant), Sphericity: ≥95% (ensures uniform force during welding), Oxide Thickness: ≤5nm (controlled by vacuum packaging).

These characteristics determine that the pelleting process must precisely control the temperature profile (peak 245±5°C), flux activity (pH 3.5-4.5), and environmental cleanliness (Class 1000 cleanroom).

1.2 The essence of the ball planting process

Regardless of the method, BGA ball planting requires three core tasks:

Positioning accuracy control: The deviation between the center of the solder ball and the center of the pad should be ≤ 15% of the ball diameter (e.g., the maximum deviation allowed for a 0.4mm ball diameter is 0.06mm), and the welding interface is formed: a 3-5μm thick Cu₆Сн.₅ intermetallic compound (IMC) layer is formed through reflow soldering, and the defect rate control is controlled at 0.1% for fatal defects such as bridging and virtual soldering The difference between the following different ball transplanting techniques is essentially the difference in efficiency and accuracy in achieving these three tasks.

2. In-depth analysis of mainstream ball planting technology

2.1 Template ball planting method: the king of efficiency in mass production scenarios

As the preferred solution for mass production, the core of the formwork pelleting method is to achieve batch positioning of solder balls through laser-machined stainless steel formwork, and the whole system complies with IPC-7525 template design specifications.

Technical principle: Using 0.1-0.15mm thick 304 stainless steel template, the opening (diameter = solder ball diameter + 0.075mm) matching the pad array is cut by ultraviolet laser, and the solder paste is accurately coated by the printing machine, and the automatic filling of the solder ball is realized by mechanical vibration. The stencil positioning uses a three-point calibration mechanism with a repeatability of up to ±10μm, ensuring that each solder ball falls accurately into its intended position.

Detailed explanation of the process

1. Solder Paste Printing Stage: Using a semi-automatic printing machine such as the Europlacer iineo, the SAC305 solder paste is evenly applied to the BGA pad at a thickness of 80±5μm. At this stage, the squeegee pressure (5±0.5N) and printing speed (20mm/s) should be controlled to ensure that the deviation of solder paste amount ≤ 10%.

2. Template alignment stage: Identify the positioning marks of the BGA substrate through the CCD vision system, align the template with the pad array, and control the positioning error within ±25μm. The gap between the template and the substrate is set to 1/3 of the diameter of the solder ball (e.g., 0.4mm solder ball corresponds to 0.13mm gap) to prevent solder ball sticking.

3. Solder ball filling stage: Automatic ball spreaders such as Parmi SG-200 evenly distribute solder balls on the surface of the template through a vibrating screen, using negative pressure to adsorb excess solder balls, and the filling rate can reach more than 99.5%. At this stage, the vibration frequency (50Hz) and amplitude (0.5mm) should be controlled to avoid the solder ball bounce misalignment.

4. Reflow soldering stage: Adopt Ramp-Soak-Spike standard temperature curves: preheating section (150°C/60s), constant temperature section (180°C/90s), and reflow section (245°C/30s), and the cooling rate is controlled within 3°C/s to ensure uniform growth of the IMC layer.

Performance advantages and limitations: Single-hour production capacity (UPH) of up to 1200 pieces, position repeatability of ±0.01mm, suitable for mass production of templates High manufacturing cost (about $500 per piece), replacement of product models requires reproduction of templates, and poor flexibility

2.2 Ball implanter method: balanced selection for small and medium batches

The ball planter method combines vacuum adsorption and optical alignment to achieve precise ball planting in medium batch production, which has unique advantages in multi-variety and small batch scenarios.

Vacuum system: using scroll vacuum pump, the vacuum level is stabilized at -80kPa~-90kPa, ensuring that small solderballs with a diameter of 0.3mm can also be reliably adsorbed, vision system: equipped with 2μm resolution CCD camera and dual telecentric lenses to realize the synchronous identification of the pad and solder balls, temperature control platform: using PID temperature control heating table, the substrate temperature is stabilized at 35±2°C, Enhanced flux activity

Key process parameters

Поток Selection: ROL0 no-clean flux (solids < 3%) with a viscosity controlled at 80-120kcp (25°C) to ensure neither bridging due to excessive fluidity nor affecting solder ball adhesion due to excessive viscosity

Ball planting pressure: Dynamically adjust according to the diameter of the solder ball, 0.3mm ball corresponds to 0.5N/ball, 0.5mm ball corresponds to 1.2N/ball, too much pressure will cause damage to the pad, too small will affect the bonding strength

Environmental control: Reflow after ball planting in a nitrogen protection atmosphere, and the oxygen content is controlled below 500ppm to reduce solder ball oxidation

Yield improvement techniques

Bake the BGA substrate at 150°C/2 hours before ball planting, control the moisture content below 0.02%, avoid bubbles during reflux, and carry out vacuum adsorption calibration every 500 pieces produced to ensure that the adsorption deviation does not exceed ±5%, and adopt a step-by-step ball planting strategy: first plant the four-corner positioning ball, confirm the position accuracy and then plant the ball in batches, which can control the positioning error to ± Within 0.05mm

2.3 Solder paste printing ball planting method: a flexible solution for R&D scenarios

The solder paste printing ball planting method directly prints the "solder ball shape" solder paste stack through a specially designed template, and forms a solder ball after reflow, which is especially suitable for rapid verification in the R&D stage.

Stepped template design: The template opening adopts a stepped structure with a wide top and a narrow bottom (upper notch + 0.1mm, lower notch - 0.05mm), using the surface tension of the solder paste to form an almost spherical solder joint, reducing the subsequent shaping process, and optimizing the characteristics of the solder paste: Type 4 tin powder (particle size 20-38μm) is selected, the mixing ratio (metal content) is controlled at 89±1%, and the thixotropic index is > 0.6, ensure good shape retention after printing, process window control: squeegee angle 60°±5°, printing speed 10-20mm/s, release speed 0.5-1mm/s, the combination of these parameters can minimize the solder paste tailing phenomenon

Common defect solutions

2.4 Manual ball planting method: emergency choice for maintenance scenarios

The manual ball planting method relies on manual operation to complete the ball planting, and is mainly used for prototype verification (<10pcs) or maintenance occasions when equipment is in short supply, and its quality control is the most difficult.

Operating specifications

ESD protection: must be equipped with an ion fan (balanced voltage <±35V), an anti-static bracelet (impedance 10⁶-10⁹Ω) and an anti-static workbench to avoid static breakdown of the chip, use an optical microscope of more than 20 times to assist positioning, and use a tungsten steel probe (0.1mm diameter) for solder ball adjustment to avoid scratching the padOperators need to pass IPC-7711/7721 CIS certification, be proficient in solder paste coating (thickness 50-80μm) and heat gun use (temperature 250±10°C), manual ball implantation usually has a fatal defect rate of 10-15%, and the risk should be reduced by the following measures: Use the "dispense first, then plant the ball" process: apply a small amount of flux (0.2mm diameter) to the center of the pad and then place thesolder ball to improve positioning accuracy Heating: The heat gun is preheated at 150°C for 30s, then heated to 245°C for 60s to reduce thermal shock, 100% X-ray inspection: Use equipment such as the Dage XD7600 to check the internal voidity rate of the solder joint, ensuring a ≤ of 25%

3. Comparison of technical parameters and selection guidelines

3.1 Comparison of core parameters of the four types of technology

3.2 Scenario-based selection suggestions

Large-scale mass production scenario

Template ball implantation + AOI inspection system, equipped with automatic optical inspection equipment such as Omron VT-S500, to achieve a defect recognition rate of 99.99%, with a dual-track printing machine configuration, to achieve a production capacity of 1500 pieces per hour, key control points: stencil cleanliness (wipe every hour), solder paste viscosity (every 4). Detect once an hour).

Multi-variety small and medium-sized batch scenarios

Ball planter method + quick change system, equipped with more than 3 sets of positioning fixtures, to achieve 5-minute quick change of different BGA models, using offline programming system, complete the ball planting program in advance, reduce downtime, key control points: vacuum adsorption calibration, optical system focal length adjustment

R&D verification scenario

Solder paste printing method + flexible template, using magnetically fixed flexible stencil, can quickly modify the opening design (suitable for parameter iteration), with a small reflow oven (e.g. ERSA HOTFLOW 2/20) to achieve small batch trial production, key control points: solder paste refrigeration conditions (5-10°C, shelf life 6 months), Warm-up time (≥4 hours).

Maintenance emergency scenarios

Recommended scheme: manual ball planting method + X-ray re-inspection, making simple positioning fixtures (accuracy ±0.1mm), improving the efficiency of manual ball planting, using local heating head (diameter 3mm) for selective reflow to avoid damage to peripheral components, key control points: operator qualifications, effectiveness of electrostatic protection measures

4. Process optimization and development trend

Common process optimization direction

Regardless of the pelleting method, the following optimizations can significantly improve quality: Pad pretreatment: Plasma cleaning (500W, 60s) to remove the oxide layer to achieve a pad contact angle of < 30°, Flux management: Precise control of the solder dose (0.005-0.01μL/pad) using a syringe dispensing valve to avoid overdose bridgingReflow curve optimization: Adjust the temperature ramp-up rate (2°C/s ≤ large devices) according to the BGA size to reduce thermal stress

Technology development trends: Intelligent ball planting system: integrating machine vision and AI algorithms to realize online identification of solder ball defects and adaptive adjustment of parameters (prediction accuracy > 95%), micro-pitch ball implantation technology: development of nanoscale positioning system (accuracy ±5μm) and low-spatter solder paste for pitches below 0.3mmLead-free advancement: Study Sn-Bi-In alloys with low melting points (melting point 170-180°C) to reduce thermal damage to the substrate

The choice of BGA pelleting technology is essentially a balance between precision, efficiency and cost. With the development of electronic packaging towards high density and high reliability, the pelleting process is shifting from "experience-led" to "data-driven", and the control goal of ppm-level defect rate is achieved through the digital management of process parameters and the quality traceability of the whole process. In the future, with the development of heterogeneous integration technology, the ball planting process will be deeply integrated with 3D packaging, hybrid bonding and other technologies, and become the core supporting process of advanced packaging manufacturing.