jaseg/phd-thesis
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

WIP

Browse source
This commit is contained in:
jaseg 2025-09-09 21:41:10 +02:00
parent 358b988a55
commit 85e348bc6e
1 changed files with 89 additions and 50 deletions
Download patch file Download diff file Expand all files Collapse all files

139
chapter-hsms/chapter.tex
View file

@ -388,6 +388,95 @@ by several hundred micrometers of the rigid PCB's substrate. The meshes on both
structure size of \qty{150}{\micro\meter}. The vertical separation between the two meshes was several times that
structure size, which limits the possible angles an attack tool could be inserted through both mesh layers.
\subsubsection{Contact and trace construction.}
Regular Printed Circuit Boards are frequently used to implement tamper-sensing meshes as shown in
Figure~\ref{hsm_fic_materials_pcb_rigid}. PCB production is a highly advanced, large-scale industry and PCBs are
inexpensive, commodity products. PCBs can be manufactured with many layers, at almost arbitrary total thickness, and
offer small structure sizes enabling the creation of fine features down to approximately \qty{100}{\micro\meter} even on
commodity processes. The primary disadvantage of using PCBs to implement tamper-sensing meshes is that PCBs are
fundamentally designed to be as robust as possible. The traces on the top of a PCB are etched from a thick (usually
\qty{35}{\micro\meter} on the outer layers) copper foil adhered to the PCB substrate. As a result, the PCB and the
traces on its surface are easy to manipulate by hand using tools like knives and techniques like soldering. For a
tamper-sensing mesh, trace patterns manufactured to be more fragile might be advantageous. Additionally, standard PCBs
are made using a rigid FR-4 fiberglass/epoxy substrate. Since a tamper-sensing mesh must often enclose all sides of a
payload, flexible foils offer benefits over rigid PCBs.
Figure~\ref{hsm_fig_materials_pcb_flex} shows a Flexible Printed Circuits (FPCs) produced in a standard commercial
process similar to PCB production. In FPCs, a copper foil adhered to a substrate is etched, but the substrate here
usually is a thin foil made from polyimide, an orange, temperature-resistant polymer that survives common reflow (hot
air) soldering temperatures. In contrast to rigid PCBs, FPCs are usually limited to no more than four layers before
losing flexibility. Flexible PCBs are often used for tamper-sensing meshes that wrap around a payload, but they come
with the same limitation as standard PCBs: Due to their robust substrate and thick copper layers, they are easily
manipulated by hand.
Both rigid PCBs and FPCs can be soldered directly to a PCB. FPCs are additionally suitable for use with standard
Zero Insertion Force (ZIF) FPC connectors. % FIXME pics
The FPC in Figure~\ref{hsm_fig_materials_pcb_flex} has a contact area for a ZIF connector on the right side, called
\emph{gold fingers} in industry terms. Soldered board-to-board connectors can be used in situations where the mesh PCB
or FPC needs to be connected at an offset or at an angle. % FIXME pics
% FIXME intro into elastomeric connectors
Elastomeric connectors as well as RF shielding gaskets can also be used with rigid PCBs or with FPCs when a rigid
backing material is used. In tamper-sensing mesh applications both have the advantage that they simultaneously serve as
an intrinsic disassembly detector since they require continuous pressure to maintain electrical contact.
Figure~\ref{hsm_fig_materials_pcb_rigid} shows a landing pattern for an elastomeric connector on a PCB.
Figure~\ref{hsm_fig_materials_silver_ink} shows an FPC created in a different process. Here, instead of
photolithographically etching a continuous copper foil adhered to a flexible substrate, the substrate is instead printed
using a conductive ink. A variety of printing processes are suitable for this technique. The conductive ink is based on
small conductive particles suspended in a hardening binder. Common conductive ink materials are silver and carbon.
Silver-based inks offer lower resistance compared to carbon-based inks, but are prone to surface oxitation and as such
are not suitable for contacts. As such, they are often combined with a carbon ink used in contact areas. Carbon-based
inks have high resistance, and can be used to create embedded resistors. The circuit shown in
Figure~\ref{hsm_fig_materials_silver_ink} contains a tamper-sensing mesh on a lower layer, and a keypad matrix with
carbon contacts on its surface.
Figure~\ref{hsm_fig_materials_gold_lds} shows part of a mesh and a contact created using Laser Direct Structuring and
electroless gold plating. Where in electroplating electrical current is used to deposit metal atoms on a surface, in
electroless plating a series of chemical reactions is used. Electroplating requires all traces to be electrically
connected to form a single electrode, while electroless plating can be used on the finished circuit. In
Figure~\ref{hsm_fig_materials_gold_lds}, it is visible how the trace was created using three parallel passes by the
laser. The micrograph also shows the rather coarse edge structure created by LDS, which is caused by the rough surface
left after pulsed laser ablation. The uneven, thin layer of metallization created by LDS results in mechanically fragile
contacts. They must be contacted using a soft material, usually an elastomeric connector.
\begin{figure}
\centering
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{trace_material_copper_pcb.jpg}
\caption{Standard photolithographic copper PCB process on rigid FR-4 fiberglass substrate}
\label{hsm_fig_materials_pcb_rigid}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{trace_material_copper_flex.jpg}
\caption{Standard photolithographic copper PCB process on flexible polyimide substrate}
\label{hsm_fig_materials_pcb_flex}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{trace_material_silver.jpg}
\caption{Screen printing process using silver ink with some carbon ink contact pads for embedded buttons}
\label{hsm_fig_materials_silver_ink}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{trace_material_contact_gold_lds.jpg}
\caption{Laser direct structuring using electroless gold plating}
\label{hsm_fig_materials_gold_lds}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{trace_material_carbon.jpg}
\caption{Screen printing process using carbon ink}
\label{hsm_fig_materials_carbon_ink}
\end{subfigure}
\caption[Mesh materials]{Materials and manufacturing processes used for mesh traces and contacts.}
\label{hsm_fig_materials}
\end{figure}
\subsubsection{3D construction.}
\begin{figure}
@ -535,56 +624,6 @@ cavity. Below this standard mesh stackup are two that are used to create a via f
Figure~\ref{hsm_fig_3d_sandwich_via_fence} in an attempt to protect the sides around the central cavity. Below these two
via fence layers, at the bottom of the PCB is one more layer containing the pads connecting it to the base PCB.
\subsubsection{Contact and trace construction.}
Contacts
Figure~\ref{hsm_fig_materials_gold_lds} shows part of a mesh and a contact created
using Laser Direct Structuring and electroless gold plating. Where in electroplating electrical current is used to
deposit metal atoms on a surface, in electroless plating a series of chemical reactions is used. Electroplating requires
all traces to be electrically connected to form a single electrode, while electroless plating can be used on the
finished circuit. In Figure~\ref{hsm_fig_materials_gold_lds}, it is visible how the trace was created using three
parallel passes by the laser. The micrograph also shows the rather coarse edge structure created by LDS, which is caused
by the rough surface left after pulsed laser ablation. The uneven, thin layer of metallization created by LDS results in
mechanically fragile contacts. They must be contacted using a soft material, usually an elastomeric connector.
Figure~\ref{hsm_fig_materials_carbon_ink}
\begin{figure}
\centering
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{trace_material_copper_pcb.jpg}
\caption{Standard photolithographic copper PCB process on rigid FR-4 fiberglass substrate}
\label{hsm_fig_materials_pcb_rigid}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{trace_material_copper_flex.jpg}
\caption{Standard photolithographic copper PCB process on flexible polyimide substrate}
\label{hsm_fig_materials_pcb_flex}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{trace_material_silver.jpg}
\caption{Screen printing process using silver ink with some carbon ink contact pads for embedded buttons}
\label{hsm_fig_materials_silver_ink}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{trace_material_contact_gold_lds.jpg}
\caption{Laser direct structuring using electroless gold plating}
\label{hsm_fig_materials_gold_lds}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{trace_material_carbon.jpg}
\caption{Screen printing process using carbon ink}
\label{hsm_fig_materials_carbon_ink}
\end{subfigure}
\caption[Mesh materials]{Materials and manufacturing processes used for mesh traces and contacts.}
\label{hsm_fig_materials}
\end{figure}
\subsubsection{Payment Terminal Construction}
\begin{figure}