Delamination in HDI PCBs

To answer the double demands of smaller geometries and greater functionality, we are seeing more and more of High Density Interconnect (HDI) PCBs in everything from smartphones to automotive systems. But for all of the advantages that HDI offers, there are also quite a few weaknesses that come with or gets augmented by the HDI format. Among these weaknesses, a good example is the problem of delamination, a problem that was known to all PCBs and IC substrates, but which, in the case of HDI, takes on greater dimensions in the smaller dimension of the device. In our article for today, we will be taking a look at this problem, its different causes and its different solutions.

Delamination in HDI PCB

Delamination is the separation of layers or interfaces within a printed circuit board. For HDI PCBs with their ultra-thin dielectrics, microvias, and sequential lamination cycles, delamination is not only more likely but also more challenging to detect and repair. The complexity and density of HDI designs create new risk points and demand a rigorous, multi-faceted approach.

The consequences of delamination for an HDI PCB are severe: open circuits, intermittent failures, decreased mechanical strength, and, ultimately, costly field failures or recalls. To ensure robust and reliable products, manufacturers and designers must confront several different causes across material choices, processing steps, and design decisions.

Material Causes

Material selection is the foundation of every HDI build. Delamination in HDI assembly results from poor material compatibility, moisture absorption, or mismatches in the coefficient of thermal expansion (CTE). To enable fine features and microvia formation, HDI structures employ specialized laminates, adhesives, and resins. If a layer ends up absorbing moisture during storage or handling, vapor expansion during lamination or reflow soldering can force layers apart, initiating delamination.

Within an HDI stackup, a critical location for material delamination is at the interface between resin-coated foils, copper layers, or adhesives within the HDI stack-up. These vulnerable spots are sensitive. Thin dielectrics and closely spaced features leave little room for error. Incompatible or improperly cured adhesives can also trigger delamination if subjected to thermal or mechanical stresses during manufacturing or operation.

There are several ways to detect these defects on your HDI PCB. Detection of material-driven delamination relies on techniques such as C-mode Scanning Acoustic Microscopy (C-SAM) and cross-section analysis. C-SAM visualizes internal separations invisible to the naked eye, while micro sectioning confirms interface quality and lamination integrity. Visual signs of delamination include blistering, separation lines, or unexpected mechanical weaknesses.

Process Causes

The HDI board is delicate, and requires a delicate manufacturing process, which in turn creates its own unique risks for delamination. Lamination involves tightly controlled cycles of temperature, pressure, and vacuum. Voids and weak bonds within the PCB stack-up can result from even minor deviations such as insufficient vacuum, temperature spikes, or uneven pressure. These voids and weak bonds end up causing delamination.

The laser-drilled microvias used in HDI designs further introduces additional risk factors. Incomplete resin fill, residues from drilling or plasma cleaning, or microcracks generated during these steps, these little defects can all act as starting points for delamination. You will want to keep an eye out for the most vulnerable spots: microvia interfaces, especially when in stacked via configurations; buried and blind vias; and the edges of the board after routing or singulation.

If you are a fabricator, you will want to enforce strict process control throughout HDI fabrication. You will want to optimize their lamination parameters for the chosen materials and verified on every batch. Pre-baking, vacuum cycles, and general cleanliness are essential to remove entrapped moisture and outgassing agents. You will also want to fine tune laser settings in order to avoid over-drilling or heat damage. Robust cleaning steps help eliminate residues from microvia walls.

Design Causes

Design choices exert a powerful influence on delamination risk in HDI PCBs. Overly aggressive via densities, excessive stacking of microvias, or minimal pad and annular ring sizes can concentrate stress and strain within critical layers. Each additional lamination cycle in a sequential HDI build adds risk of delamination, particularly if the stack-up is not designed with manufacturability in mind.

A complex HDI stack-up with a system of vias of different types and thin cores, will always be pushing the boundaries of mechanical and thermal stability. If your pad or via geometries take too much liberty with recommended tolerances, you can very well compromise your resin/adhesive interfaces, finally resulting in delamination after thermal cycling or flexing in use.

There are established tricks and practices to prevent these design-related issues of HDI delamination. You will want to use staggered microvia configurations wherever feasible. You will want to consult your manufacturer guidelines for minimum annular ring and pad sizes, and avoid unnecessarily complex stack-ups that will require excessive sequential lamination. An early engagement with a trustworthy PCB fabricator is never a bad idea: with constructive communication, your PCB fabricator can provide helpful design-for-manufacturability input and suggest tweaks that will not only bring down production costs by improving yields, but also improve the reliability of your product.

Conclusion

In our article for today, we revisited the problem of delamination, an old problem known to all kinds of PCBs, but seriously accentuated in the context of HDI. This problem arises and can be tackled in different parts of the entire production process: some errors can be resolved with intelligent designs, some with careful material selection, some with good practices on the HDI assembly line. To tackle delamination, a collaborative effort is necessary from both the part of the designer and the part of the manufacturer. We hope this article on delamination in HDI PCBs was helpful to you, and was able to prevent delamination problem in your own project. We look forward to seeing you next time.