PCB Glob Top: How COB Encapsulation Protects Bare Silicon Dies
What This Video Covers
A black blob on a printed circuit board may be a PCB glob top, a protective resin covering a bare semiconductor die and its electrical connections in a chip-on-board (COB) assembly. Unlike a conventional integrated circuit housed in a separate package, COB attaches the bare silicon die directly to a PCB or substrate. In traditional wire-bonded designs, fine metal wires connect the die's bonding pads to conductive pads on the board, and the glob top protects these delicate connections from physical damage and environmental contamination.
This packaging approach can reduce component height and save board space in compact electronics, including IoT devices, consumer products, and industrial control systems. However, its suitability depends on assembly requirements, reliability expectations, inspection methods, and repairability. Engineers should consider die attachment, wire-bond clearance, encapsulant compatibility, and mechanical stress during design and manufacturing.
For projects requiring bare-die integration, evaluate the complete PCB assembly and PCBA service alongside the board's fabrication requirements. An FR4 PCB may suit many conventional applications, while material selection for specialized assemblies depends on thermal, electrical, and mechanical demands. Before production, a PCB prototype can help validate component placement, assembly feasibility, and the overall design.
Key Highlights
- COB structure: A bare silicon die is mounted directly on a PCB or substrate, with fine wire bonds connecting the die to board-level electrical connections.
- Glob top protection: A protective resin dome covers the die and wire bonds, helping prevent physical damage and contamination.
- Design trade-off: COB can reduce package height and save space, but bare-die integration may complicate inspection, rework, sourcing, and replacement compared with packaged ICs.
How Chip-on-Board Assembly Works
Chip-on-board assembly integrates an unpackaged semiconductor die directly onto a substrate instead of using a conventional molded IC package. This approach can simplify the component footprint and reduce overall assembly height, making it useful when enclosure space or product dimensions are constrained.
In a traditional wire-bonded COB assembly, the die is attached to a prepared mounting area, and fine metal wires connect its bonding pads to corresponding conductive pads on the board. The electrical path depends on correct die placement, compatible bonding surfaces, stable wire geometry, and sufficient clearance around the bonding area. The exact process depends on the die design and the assembly supplier's equipment and capabilities.
After electrical connections are established, a liquid encapsulant is dispensed over the die and wire bonds and then cured according to the material manufacturer's requirements. This protective resin forms the glob top. Its coverage must protect the vulnerable connections without interfering with neighboring components or other board features.
For manufacturing teams, the important distinction is that COB is a semiconductor assembly method, not simply a type of PCB fabrication. Standard PCB processes such as etching, drilling, plating, and lamination prepare the board, while die attachment, wire bonding, and encapsulation require specialized assembly processes. Not every conventional SMT assembly line is equipped to perform bare-die COB.
PCB Glob Top Materials and Manufacturing Considerations
Glob top materials are commonly formulated resins selected to protect the die and its electrical connections. Depending on the application, selection criteria can include adhesion, flow behavior, curing requirements, moisture resistance, thermal expansion, and compatibility with the substrate and adjacent materials. Epoxy-based materials are commonly used, but the appropriate formulation depends on the assembly design and operating environment.
The resin must flow around the die and wire bonds while maintaining adequate coverage. If it flows too far, it may contaminate nearby pads, interfere with neighboring components, or extend beyond the intended keep-out area. If coverage is incomplete, delicate wires may remain exposed. Excessive dispensing pressure, unsuitable viscosity, trapped air, or an incompatible curing profile can also contribute to defects.
Manufacturing teams should define the encapsulation area and allowable glob top profile during design. This includes checking the distance to adjacent components, enclosure clearance, test pads, and any surfaces that must remain accessible. The dispensing process should be validated using the actual board geometry and encapsulant rather than relying solely on nominal dimensions from the layout.
Thermal and mechanical reliability also deserve attention. Differences in thermal expansion between the resin, silicon die, bond wires, and PCB substrate can create stress during temperature changes. Material selection and curing conditions should therefore reflect the intended operating environment, particularly for automotive electronics, industrial control systems, and other products exposed to repeated thermal cycling or vibration.
Glob Top Manufacturing Checks
| Manufacturing check | Potential issue | Recommended practice |
|---|---|---|
| Resin dispensing | Incomplete coverage or excessive overflow | Validate dispense volume, material flow, and coverage on representative assemblies |
| Wire-bond protection | Wire deformation or exposed connections | Maintain suitable clearance and avoid disturbing the bonded area during dispensing |
| Curing process | Poor adhesion or incomplete cure | Follow the material supplier's cure requirements and validate the process |
| Thermal compatibility | Stress at interfaces or bond-wire damage | Evaluate material compatibility and validate reliability under expected conditions |
| Visual inspection | Voids, contamination, or irregular encapsulation | Establish inspection criteria and use additional test methods where needed |
COB vs. Packaged ICs: Design and Sourcing Trade-Offs
COB is not automatically better than conventional packaged IC assembly. It is a packaging choice that can offer advantages when space, height, or product integration is important, but it also changes how the device is sourced, assembled, inspected, and serviced.
A packaged IC typically arrives with a standardized external package and leads or solderable terminals. This can make component sourcing, automated SMT placement, board-level inspection, and replacement more straightforward. In contrast, COB requires access to compatible bare dies and a suitable die-attachment, wire-bonding, and encapsulation process.
The black resin dome also affects maintenance decisions. Once the die and wires are encapsulated, direct visual inspection and physical access to the underlying connections become limited. Depending on the failure and product construction, repair may be impractical or uneconomical. Design teams should account for this before committing to COB in products that require field servicing or component-level replacement.
For compact consumer electronics, low-profile modules, and selected high-volume products, COB can be a reasonable choice when its integration benefits outweigh the manufacturing and serviceability constraints. For products that depend on standardized components, easy sourcing, or straightforward rework, a packaged IC may be the more practical option.
Selection Guide: COB vs. Packaged IC
| Decision factor | Chip-on-board with glob top | Conventional packaged IC |
|---|---|---|
| Package height | Can be lower, depending on the design | Depends on the selected IC package |
| Board space | Can reduce the footprint associated with separate packages | Depends on package dimensions and layout |
| Assembly requirements | Requires compatible bare-die attachment, interconnection, and encapsulation capabilities | Commonly compatible with established SMT processes |
| Inspection and rework | Encapsulation limits access to the die and wire bonds | Often easier to inspect and replace at board level |
| Component sourcing | Requires suitable bare-die supply and process compatibility | Often benefits from established packaged-component supply channels |
| Best-fit considerations | Space-constrained designs with validated assembly processes | Designs prioritizing standardized sourcing and serviceability |
DFM Considerations for Reliable COB Assembly
Design for manufacturability (DFM) for COB starts before the PCB layout is finalized. The engineering team should confirm that the assembly supplier can support the selected die, bonding method, pad geometry, substrate finish, and encapsulant. Bare-die assembly requirements can differ substantially from those of a standard SMT project, so early communication prevents avoidable design changes after fabrication.
The board layout should reserve sufficient space for die attachment, wire-bond loops, glob top coverage, and any dispensing or inspection access required by the manufacturing process. Keep-out areas should account for the actual resin profile, not just the silicon die outline. Nearby components, connectors, exposed test pads, and enclosure walls can all constrain the available space.
Material and surface compatibility are equally important. The die-attach material, bonding interface, substrate, and encapsulant must work together under the expected electrical, thermal, and mechanical conditions. A layout that is electrically correct may still be unsuitable for manufacturing if the bond pads are inaccessible, the resin cannot be dispensed consistently, or the final assembly exceeds the available enclosure height.
Before volume production, a prototype or engineering build should validate the actual assembly flow. Review die placement, wire-bond quality, encapsulation coverage, electrical test results, and any application-specific reliability requirements. For products exposed to vibration, humidity, or temperature cycling, qualification should be based on the product's intended use and an agreed test plan rather than a generic assumption that the resin alone guarantees reliability.
A practical DFM review should also define acceptance criteria and failure analysis procedures. For example, suspected wire-bond failures beneath a glob top may require electrical diagnostics or specialized analysis because the connections are not directly accessible. Clear inspection criteria, traceability, and agreed repair-or-reject rules help engineering and procurement teams evaluate manufacturing risk before committing to production
FAQ
Q1: Can a PCB glob top be removed to repair the chip underneath?
A1: Removing a glob top can damage the bare die, fine wire bonds, or substrate, particularly when the resin adheres strongly to the underlying assembly. Repair feasibility depends on the encapsulant, die construction, and failure location. For many products, replacing the assembly or PCB is more practical than attempting to expose and repair the chip.
Q2: Does a PCB glob top improve thermal management?
A2: A glob top primarily protects the die and its electrical connections; it should not be assumed to improve heat dissipation. Thermal performance depends on die power, attachment materials, substrate construction, heat-flow paths, and the surrounding enclosure. Thermal analysis and validation are necessary when the semiconductor generates significant heat.
Q3: What manufacturing capabilities are required for COB assembly?
A3: COB typically requires compatible bare-die handling and attachment, wire bonding or another suitable interconnection method, controlled resin dispensing, curing, and appropriate inspection and electrical testing. The required capabilities depend on the die, substrate, product specifications, and qualification requirements. Standard SMT capability alone does not establish that a supplier can manufacture COB assemblies.
Q4: Is COB with glob top suitable for automotive or industrial electronics?
A4: It can be suitable when the assembly is designed and qualified for the application's operating conditions. Engineers should evaluate temperature cycling, vibration, moisture exposure, electrical performance, and long-term material compatibility as applicable. Supplier process controls and product-specific reliability testing should be established before production approval.
What's hiding under that black blob on a PCB?
It might be a COB assembly, or Chip on Board.
Unlike a conventional IC, COB mounts a bare silicon die directly onto a PCB or substrate.
So, how does it connect to the circuit?
In traditional wire-bonded COB, tiny metal wires connect the chip's bonding pads to pads on the board.
That black dome is called glob top.
Usually made from protective resin, it covers the bare die and delicate wires, helping shield them from physical damage and contamination.
Why use this design?
COB can save space and reduce package height, but conventional packaged ICs are often easier to source and replace.
Here's a useful tip: Don't judge a chip by its appearance alone.
A black blob may hide an entire integrated circuit and its connections.
