The FPC, also known as the Flexible Printed Circuit Board, has a significantly different assembly and soldering process compared to rigid circuit boards. This is because the FPC’s flexibility and softness require special carrier boards for fixation and transmission. Without these, basic SMT processes like printing, mounting components, and reflow soldering cannot be completed.
Pre-treatment of FPC:
FPCs, being flexible, often absorb moisture during transportation and storage due to their softness. Pre-baking is necessary before SMT assembly to gradually force out absorbed moisture. Failure to do so might result in problems like delamination or bubbling during reflow soldering. Pre-baking typically involves temperatures of 80-100℃for 4-8 hours, although higher temperatures can be used with shorter durations after testing. It’s essential to confirm FPC’s tolerance to the set baking temperature through trials or consultation with the manufacturer.
Production Process:
Special Carrier Board Production: These boards are manufactured based on CAD files, incorporating high-precision positioning templates to match the holes on the FPC. Variations in FPC thickness may require custom modifications in the board to ensure flatness during printing and component mounting.
Fixation of FPC: Using specialized techniques like single or double-sided high-temperature adhesive tape to secure the FPC precisely onto the carrier board.
Solder Paste Printing: FPCs demand solder pastes with excellent printability and adhesion due to their non-uniform surface. Unique considerations like using polyurethane scrapers instead of metal ones are essential.
Component Mounting: High-speed placement machines are used, with a focus on precision due to potential unevenness in the FPC surface.
In the assembly and soldering process of flexible electronics in PCBA, precise positioning and fixation of the FPC are pivotal, with the key lying in creating suitable carrier boards. Subsequently, pre-baking, printing, component mounting, and reflow soldering of the FPC follow suit. Clearly, the SMT (Surface Mount Technology) process for FPCs is significantly more challenging than that for rigid PCBs. Hence, precise configuration of process parameters is essential. Simultaneously, rigorous production process management holds equal importance. It’s imperative to ensure strict adherence by operators to every provision in the Standard Operating Procedures (SOP). Line engineers and IPQC (In-Process Quality Control) must intensify inspections, promptly identifying any abnormal situations on the production line, analyzing causes, and taking necessary measures. This approach is necessary to keep the defect rate of the FPC SMT production line within several dozen parts per million (PPM).
Reflow Soldering: Employing forced convection infrared reflow ovens to ensure even temperature distribution across the FPC, preventing defects like tilted solder pads or solder ball formation.
Inspection, Testing, and Cutting:
After assembly, thorough visual inspection is crucial for issues like residual adhesive, discoloration, solder residues, or component defects. Automated Optical Inspection (AOI) might not be suitable due to FPC surface irregularities. Testing using ICT (In-Circuit Test) or FCT (Functional Circuit Test) might require dividing the FPCs. Specialized cutting tools or dedicated FPC stamping/cutting molds are recommended for efficient and high-quality separation.
The intricate nature of FPC assembly demands precise handling and specialized equipment to ensure the quality and functionality of the final product.
Basic equipment required for PCBA production includes solder paste printers, pick-and-place machines, reflow soldering machines, AOI (Automated Optical Inspection) testers, component lead trimmers, wave soldering machines, solder ovens, board washing machines, ICT (In-Circuit Test) fixtures, FCT (Functional Circuit Test) fixtures, aging test racks, and more. Different scales of PCBA processing plants might have varying equipment configurations.
PCBA Production Equipment
Solder Paste Printer:
Modern solder paste printers consist of mechanisms for fixing boards, applying solder paste, printing, and board transportation. The working principle involves securing the circuit board to the printing positioning table. Solder paste or red glue is then applied onto corresponding pads via the left and right blades of the printer through a steel mesh. The PCB with even paste application is then transferred to the pick-and-place machine for automatic component mounting.
Pick-and-Place Machine:
Also known as a “mounting machine” or “surface mount system,” it accurately places surface-mounted components onto PCB pads by moving the mounting head. It comes in both manual and fully automatic variants and is positioned after the solder paste printer in the production line.
Reflow Soldering:
This process involves a heating circuit within the reflow equipment that directs heated air or nitrogen at a sufficiently high temperature onto the assembled components on the board. This melts the solder on the component sides, bonding them to the main board. Its advantages include temperature control, oxidation avoidance during soldering, and easier manufacturing cost control.
AOI Inspection Equipment:
AOI (Automatic Optical Inspection) is an optical-based equipment used to detect common defects encountered in soldering production. It’s a rapidly developing testing technology where the machine scans PCBs using cameras, compares solder points with the database’s valid parameters, identifies defects through image processing, and displays or marks them for repair personnel.
Component Lead Trimming Machine:
Used for trimming and shaping the leads of through-hole components.
Wave Soldering:
Involves direct contact between the soldering surface of the through-hole board and high-temperature liquid solder to achieve soldering. The high-temperature liquid solder maintains a slanted surface, creating a wave-like phenomenon using special equipment, hence the term “wave soldering.” The main material used is solder bars.
Solder Oven:
Generally used as a soldering tool in electronic soldering. It provides good consistency for soldering discrete component PCBs, convenient operation, and high work efficiency.
Board Washing Machine:
Cleans PCBA boards, removing residues post-soldering.
ICT Testing Fixture:
ICT Test primarily uses test probes to contact the test points laid out in the PCB layout to check for PCBAs‘ circuit continuity, shorts, and all components’ soldering conditions.
FCT Testing Fixture:
FCT (Functional Test) refers to a method of providing a simulated operating environment for the Unit Under Test (UUT) to validate its functionality in various design states by applying appropriate stimuli and measuring the response at output terminals.
Aging Test Rack:
Conducts batch testing of PCBA boards by simulating prolonged user operations to identify problematic boards.
PCBA outsourcing refers to the practice where PCBA manufacturers subcontract PCBA orders to other capable PCBA processing factories. So, what are the typical requirements for PCBA outsourcing?
Bill of Materials (BOM):
Strict adherence to the BOM, PCB silk screen, and outsourcing requirements for component insertion or placement.
Any discrepancies between materials, the BOM, PCB silk screen, conflicting process requirements, or unclear requirements preventing operation should be promptly communicated to confirm material and process correctness.
Anti-Static Requirements:
All components treated as static-sensitive devices.
Personnel in contact with components wear anti-static clothing, use anti-static wristbands, and wear anti-static shoes.
Static-sensitive components in raw material procurement and storage phase are packaged in anti-static packaging.
Use of anti-static workbenches during operations; components and semi-finished products stored in anti-static containers.
Reliable grounding of soldering equipment; use of anti-static soldering irons, pre-checking before use.
Storage and transportation of PCB board semi-finished products in anti-static boxes, isolation materials utilizing anti-static pearl cotton.
Complete devices without casings use anti-static packaging bags.
Component Appearance Marking for Insertion Orientation:
Polarized components inserted according to polarity.
For side-labeled components (e.g., high-voltage ceramic capacitors) inserted vertically, the label faces right; horizontally, the label faces downward. For top-labeled components (excluding SMD resistors), horizontally, the font aligns with the PCB silk screen; vertically, the font’s top faces right.
Soldering Requirements:
Inserted components should have pin heights of 1.5–2.0mm on the soldering surface. Surface-mounted components should be flat on the board surface, with smooth, slightly arched solder joints. Solder should cover 2/3 of the joint height without surpassing it. Insufficient solder, spherical solder joints, or solder covering SMDs are considered defects.
Solder joint height: For single-sided boards, solder attachment to pins should be no less than 1mm; for double-sided boards, no less than 0.5mm with proper penetration.
Solder joint shape: Conical and covering the entire solder pad.
Solder joint surface: Smooth, bright, free from black spots, flux residues, burrs, pits, air holes, exposed copper, or defects.
Solder joint strength: Proper wetting with solder pads and pins, free from cold soldering or false soldering.
Solder joint cross-section: Component leads shouldn’t be trimmed into the solder portion; no solder cracking at the contact surface between pins and solder; no burrs or reverse hooks at the junction.
Connector soldering: Bottom-mounted connectors with proper positioning, direction, and post-soldering height not exceeding 0.5mm or deviation from silk screen frames for aligned rows.
Transportation:
To prevent PCBA damage during transport, the following packaging should be utilized:
Container: Anti-static transit boxes.
Insulation material: Anti-static pearl cotton.
Spacing: A gap greater than 10mm between PCBs or between the PCB and the box.
Elevation: Ensure a space greater than 50mm from the top of the transit box to avoid pressing against power sources, especially when wires are present.
Board Cleaning Requirements:
Clean board surface, free from solder balls, component pin residues, or stains. Particularly for solder joints on plug-in surfaces, no visible soldering residue should be present.