1. Introduction to Aluminum-Core PCBs
What Is a PCB?
A PCB, short for printed circuit board, is also known as a printed wiring board. In electronic equipment, the PCB provides both the mechanical support for electronic components and the electrical interconnections between them.
PCB Classification
By product structure, PCBs can be classified as rigid boards, flexible boards, rigid-flex boards, HDI boards, and package substrates.
By the number of routing layers, PCBs can be classified as single-sided boards, double-sided boards, and multilayer boards.
What Is an Aluminum-Core PCB?
MCPCB stands for metal-core printed circuit board. It uses a metal substrate with better thermal conductivity than a conventional PCB to improve heat dissipation. Aluminum-core PCBs are one of the most common MCPCB types. They efficiently transfer heat away from electronic components, helping improve overall product performance and thermal reliability.
An aluminum-core PCB is a metal-clad board with effective heat-dissipation capability. A typical single-sided board consists of three layers: the circuit layer, usually copper foil; the dielectric layer; and the aluminum base layer.
Applications requiring components on both sides can use a double-sided structure consisting of a circuit layer, dielectric layer, aluminum core, dielectric layer, and second circuit layer. In a small number of applications, a multilayer structure can be formed by bonding a conventional multilayer PCB to an aluminum core through dielectric layers.
Aluminum-core PCBs are widely used in LED lighting products. One side is generally white and is used for soldering LED leads, while the opposite side exposes the aluminum or is coated with a thermally conductive compound before contacting a heat-dissipation structure.
Common Aluminum-Core PCB Structures
Single-Sided Component Mounting with Double-Sided Routing
This type of aluminum-core PCB has two routing layers and dielectric layers. Heat travels from the component to the first routing layer, through the first dielectric layer, to the second routing layer, through the second dielectric layer, and finally to the aluminum layer.
Double-Sided Component Mounting with Double-Sided Routing
When component placement on only one side cannot satisfy the design requirements, an aluminum-core PCB can be designed for component mounting on both sides. In this stackup, the upper and lower routing layers must pass through the aluminum core. Because aluminum is electrically conductive, the vias require insulation treatment, such as filling the regions around the vias with insulating resin.
Heat generated by the components transfers to the routing layer, then through the dielectric layer, and finally to the aluminum core in the middle of the board. The electrical insulation of the core and the thermal path must both be considered during stackup and via design.
2. Aluminum-Core PCB Manufacturing Process
Material Cutting
The aluminum-core laminate is cut into production panels according to the required dimensions and process layout.
Circuit Formation
The copper surface is pretreated before coating. A photosensitive material is then applied, followed by circuit exposure and development. The unwanted copper is removed during etching, and the remaining resist is stripped.
The circuit is inspected by automated optical inspection, or AOI, to identify pattern defects and other visible abnormalities.
Solder Mask and Marking
Before solder mask application, the circuit surface is pretreated. Solder mask is then coated, exposed, and developed to protect the circuit while leaving the required soldering areas open. Component reference designators and other markings are subsequently printed, followed by post-curing.
Drilling, Routing, and Electrical Testing
Target positioning establishes the reference for subsequent machining. Open-and-short testing checks circuit continuity and isolation. The board is then routed or V-scored according to the required outline. High-voltage testing verifies the insulation capability between electrically isolated conductors.
Surface Finish, Inspection, and Packing
OSP, or organic solderability preservative, is applied to protect exposed copper and maintain solderability. Final quality control checks the finished boards before they are packed and placed into storage.
3. Common Aluminum-Core PCB Problems
White Solder Mask Covering Pads or Adhesive Dots
Common causes include:
- Board deformation prevents the film from fully contacting the board during exposure, causing light leakage and incomplete development.
- Film abnormalities or insufficient exposure result in incomplete pattern transfer.
Poor Legend Printing
Legend-printing defects can be caused by an incorrect screen mesh count, which causes ink accumulation and blurred markings. Other causes include failure to clean the screen promptly after equipment shutdown and continued use of a screen beyond its controlled service life, resulting in ink leakage.
Damaged Locating Holes
Locating-hole damage may result from abnormal punching dies, excessive die life, or loose equipment. Die management should be checked carefully. Secondary damage can also occur during punching, when the workpiece is lifted by the punch or shifts while being positioned in the die.
Surface Dents and Press Marks
Insufficient control of die life and punching equipment can leave punched-board debris on the work surface, producing dents or pressure marks. Inadequate machine cleaning can produce the same type of defect.
Residual Aluminum Particles
Aluminum particles may remain when the board is not cleaned after V-scoring. Particles can also remain on the worktable after final inspection and depanelization, then adhere again to the board surface. Cleaning frequency should therefore be controlled and maintained.
Yellowing After Reflow
Post-reflow yellowing may be caused by selecting an unsuitable ink grade. It can also result from a missed OSP process or from stacked boards during OSP cleaning, which prevents effective cleaning of the surface.
