What is the lay up and bond process in multilayer PCB?

Introduction

Printed circuit boards (PCBs) form the backbone of most modern electronic devices. From smartphones to laptops to industrial equipment, PCBs provide the platform to mount and interconnect electronic components. A multilayer PCB consists of multiple conductive layers separated by insulating dielectric layers. The conductive layers are used to route signals between components, while the dielectric provides insulation and prevents shorting between the conductor layers.

The lay up and bond process is a critical manufacturing step that builds up the multilayer PCB by laminating together all the layers accurately. This process ensures proper alignment, bonding, and electrical connectivity between the layers. Understanding the lay up process provides insight into how complex multilayer PCBs are fabricated.

Lay Up Process Basics

The lay up process assembles and bonds together sheets of conductive copper and insulating dielectric material in a stack-up arrangement to produce a multilayer board.

The key steps are:

• Inner layer preparation - Inner layers are first processed individually. This involves etching the desired circuit pattern into the copper cladding of a dielectric substrate.
• Lay up - The inner layers are stacked in sequence with additional dielectric core sheets. Alignment is critical.
• Bonding - The lay up is bonded under heat and pressure to fuse the layers into a single board.
• Outer layer processing - The external layers are imaged and etched to complete the multilayer PCB.

The quality and precision of the lay up directly impacts the reliability of connections between the stacked layers in the finished board.

Inner Layer Preparation

The fabrication of the inner conductive core layers involves several steps.

Dielectric substrate

• The substrate serves as the insulating foundation
• Common materials are FR-4, polyimide, ceramic

Copper cladding

• Layer of copper foil ranging from 0.5 oz to 6 oz thickness
• Foil is laminated to one or both sides of the substrate

Imaging

• Photoresist coating is applied and imaged to transfer the circuit pattern
• Can be done with traditional photo lithography or direct laser/inkjet imaging

Etching

• Unwanted copper is chemically etched away to leave only the desired circuit pattern on the substrate

Stripping and cleaning

• Residual photoresist is stripped away
• Oils and contaminates are cleaned from the surface

This process generates finished inner core layers ready for lay up. The layers can have circuits on one or both sides. High accuracy and precision in the inner layer fabrication is essential for reliable interconnections between layers.

Layer Stack Up

The layer stack up defines the sequence and orientation of the conductive and dielectric layers to produce the functionality of the multilayer PCB. Stack up planning involves:

• Which layers will have signal routing and component pads
• Power and ground reference plane locations
• Stack up sequence of cores and prepregs
• Total thickness to meet electrical and mechanical needs

Signal routing layers - Used for routing traces to connect components. Place on outer layers for accessibility.

Plane layers - Solid copper sheets for power and ground. Place between signal layers for shielding and controlled impedance environments.

Dielectric cores - Thicker insulating substrates used as the primary foundation. FR-4 or high performance polymers.

Prepregs - Thin resin coated sheets. Bonded between cores during lamination to bond the stack up.

Careful stack up design is required for optimal high speed design, signal integrity, and power distribution.

Layer Alignment

Aligning the layers accurately is critical in the lay up process. Small misalignments can cause open or intermittent connections between layers, leading to circuit failures.

Alignment methods:

• Tooling holes - Holes punched in each layer are stacked on precision tooling pins. The most common method.
• Fiducials - Register marks on each layer are optically aligned. Requires vision systems.
• Sequential lamination - Interlayer connections are etched slightly offset. Each added layer self-aligns sequentially.

Advanced processes and equipment are used to achieve alignment accuracy within 0.1mm or better for high density multilayer PCBs.

Bonding Process

Bonding fuses together the layered stack up into an integrated board through lamination. This involves subjecting the lay up to high heat and pressure in a hydraulic press. Typical lamination parameters are:

• Temperature - 180-200°C
• Pressure - 300-500 psi
• Time - 1-3 hours

The combination of heat, pressure, and resin flow from the prepregs causes the layers to bond together. The resin cures into a solid infused composite.

Vacuum assisted lamination removes air pockets for uniform bonding across large panels. Multiple board panel sizes can be processed together for efficiency.

Post-bond Processing

After lamination, additional processing completes the multilayer board fabrication:

Outer layer imaging - Apply photoresist and transfer the outer layer circuit patterns using lithography.

Outer layer etching - Etch away unwanted copper to form the exterior circuit layers.

Hole drilling - Drill holes for component mounting and interlayer connections. Accurate registration is critical.

Plating - Electrolytically plates copper over the walls of drilled holes to form interconnects between layers.

Final finish - Apply soldermask, silkscreen, and edge connectors.

Rigorous testing follows to verify electrical connectivity and board function prior to shipment.

Lay Up Process Challenges

The multilayer lay up process faces inherent challenges including:

• Maintaining layer to layer alignment and registration
• Preventing interlayer voids or resin starvation defects
• Dealing with thickness variations in dielectric materials
• Achieving uniform heat and pressure during lamination
• Handling panels with increasing area as board sizes grow

Careful process engineering and control is required to hold tolerances and produce high yields for complex multilayer PCBs.

Summary

The lay up and bond process is a critical fabrication step for multilayer PCBs. It provides the foundation to build up complex boards by accurately stacking and bonding conductive core layers separated by dielectric materials. Stringent process control over layer preparation, alignment, lamination, and drilling ensures reliable electrical connectivity and performance in the finished multilayer circuit board assembly.

Frequently Asked Questions

Here are answers to some common questions about the multilayer PCB lay up process:

What are some key differences when laying up double-sided vs multilayer boards?

For double-sided boards, alignment is only required when imaging the outer layers. With multilayers, alignment is critical during lay up and bonding of the inner core layers. More process controls are required for the layer stack up and lamination of multilayers.

How are very small vias and microvias integrated into the lay up process?

Microvias require sequential lamination where each layer is added in succession rather than all at once. This allows drilling and plating the vias from alternating sides to produce reliable interconnects.

Why is resin flow important during the bonding process?

Controlled resin flow ensures the resin fully fills between the layers, eliminating voids. The properties of the resin are engineered to flow uniformly before curing for optimum layer bonding.

How does the increasing density of circuits impact the lay up process?

Tighter tolerances are required on layer alignment, lamination uniformity, and drilling accuracy as circuit features shrink. Complex boards may also utilize buried and blind vias for greater interconnect density.

What are the main failure modes if the lay up process is not controlled properly?

Insufficient bonding can lead to delamination during board fabrication or reliability issues in the end product. Misalignment causes open circuits between layers. Voids or poor resin fill will also produce defects.