Medical Ultrasound FPCB Design: Optimization of 3L Stepped and Cutout Flexible Circuit Structures
A medical ultrasound flexible circuit design often must fit dense probe electronics into very small housings. A 3L stepped structure combined with a cutout flexible circuit layout improves spatial arrangement, reduces component interference, and protects hollow finger areas from cracking. This guide covers the process routes used for an ultrasound FPCB stepped cutout build, the wrinkling defects those routes address, and the checks needed to qualify a stepped flex circuit design for volume production.
Development Trends of FPC in Medical Ultrasound Devices
Ultrasound probes are widely used in medical diagnostic equipment. Some probes are single-use consumables, so market demand stays strong. Europe and the United States show the largest potential for these products.
FPC boards for medical ultrasound products now follow clear trends:
- Miniaturization
- High wiring density
- High interconnectivity
- Enhanced structural strength
- Efficient heat dissipation
- Ease of assembly
However, leading medical customers keep proposing new requirements. They want compact medical ultrasound FPCB designs with specialized structures. Therefore, miniaturization and high-density integration are an inevitable direction for the industry.
Existing manufacturing capability cannot yet support every specialized structure. That gap is exactly what the two structure families below address.
Stepped Flex Circuit Design for 3L Structures
What a Stepped FPC Structure Solves
In traditional PCB designs, all components sit on one plane. That layout can cause interference between components. It can also leave too little room for routing.
A stepped flex circuit design fixes both problems. It places components on different levels, so the spatial arrangement improves and interference drops. In addition, the stepped shape fits the mechanical envelope of modern probes.
Process Solutions for 3L Stepped FPC
Two technical approaches were studied for 3-layer (3L) stepped FPC boards:
1. Screen-printed solder mask with peelable adhesive
2. Carrier film lamination with controlled-depth coverlay removal
Both routes test the same question. Can the product structure be built reliably, and can manufacturing yield improve? The work also targets wrinkling defects in the stepped areas.
Cutout Flexible Circuit Medical Boards: Single-Sided Hollow Finger FPC
Flatness and Deformation in Hollow Areas
For single-sided hollow FPC boards, the finger pattern inside the hollow area keeps high flatness. The combination of hollowing and routed finger separation produces low deformation. Consequently, the board meets the connection requirements at both ends and improves product reliability.
Process Development for Cutout Flexible Circuit Medical Boards
Two process routes were developed for single-sided hollow boards. Both aim to stop finger cracking or bending in hollow areas.
Route one applies PI windowing before copper lamination. Next, dry film goes on the circuit areas. Then anti-plating dry film protection covers the finger areas.
Route two applies PI windowing after etching. Next, the coverlay is laminated. Then anti-plating dry film protection covers the finger areas.
Each route changes the order of operations. Therefore, each one carries a different risk profile in the hollow region.
Experimental Section
Mechanism Analysis and Experimental Design of 3L Boards
The study starts with mechanism analysis of the 3L stepped board. The team then designed experiments around the two process routes described above. Each route was evaluated for structure feasibility under different processes.
In addition, the experiments tracked wrinkling behavior in critical areas. Wrinkling is the main defect that limits yield on these boards.
Validation of Hollow FPC Structures
For the single-sided hollow boards, the experiments validated the manufacturing process for hollow FPC structures. The focus stayed on finger integrity and flatness inside the cutout region.
Experimental Results and Analysis
Figure 1: Step Structure and Stress Analysis
The feasibility of forming 3L stepped flexible circuits was verified through the following three technical approaches to address creasing defects in critical areas.
Technical solutions
As shown in the technical solutions in Figure 2:
Solution A:
- Form the steps first, then laminate.
- mill grooves on the solder surfaces L3 and L4;
- screen-print a peelable adhesive to provide a support structure;
- After electroplating, remove the film and clean the peelable adhesive.
Solution B:
Laminate first, then perform controlled-depth milling of the step.
A peel-off cover method applies a load-bearing film to critical areas on the TOP side and high-temperature tape to Layer L2.
The process removes the peel-off cover at a controlled depth after electroplating but before contouring.
Solution C:
Replace red copper with high-strength titanium-copper (TiCu) or beryllium-copper (BeCu).
Although this option is more costly, it serves as a backup solution.
Figure 2: 3L step flexible substrate technology roadmap
Specific Design Plan for Screen-Printing Peelable Adhesive:
- Mill grooves on the core boards of layers L3 and L4; screen-print the peelable adhesive (yellow peelable PI adhesive###) twice on layer L2 of the layer 101 core board;
- The width of the peelable adhesive should be 0.20 mm narrower on one side than the step area.
- Add a laser-cut outline to the outer layers.
- After electroplating, add a film removal process to remove the peelable adhesive.
The theoretical milling groove depth is 71 μm, so two screen prints match the groove depth.
Screen printing parameters for the peelable adhesive: printing speed 10 cm/s, mesh count 41T, wet film thickness 40–45 μm.
Specific design for depth-controlled peel-off:
- Apply a carrier film to the critical areas on the top surface of Layer 101, extending at least 1.0 mm beyond the step region on each side for support.
- apply high-temperature tape to Layer L2 (###);
- After the first laser contouring on Layer 000, add a UV depth-control process on the solder side (performing depth control and opening on two edges within the board), followed by peel-off.
- Add a carrier film removal process after electrical testing, but before contouring.
Mechanism Analysis and Experimental Design of Single-Sided Perforated Flexible Circuit Boards
Figure 3 shows the structural design of the perforated board. The board thickness is 0.18 mm, with product dimensions of 64.26 × 40.18 mm.
The perforated finger dimensions are 0.736 ± 0.1 mm × 0.2 ± 0.05 mm. The PI thickness is 50 μm, and the copper thickness is ≥35 μm.
The fingers in the perforated area are prone to breaking or bending, making it difficult to ensure positional accuracy.
Since the perforated finger area lacks support and structural reinforcement, the forming process and machining operations likely cause finger fractures or bending.
Therefore, two process routes provide solutions: one cuts the PI window with a laser before circuit fabrication, and the other fabricates the circuit before laser cutting the PI window.
This ensures that the open-frame fingers do not break or bend, verifies the feasibility and quality performance of the manufacturing process, and determines the optimal process route for the open-frame finger FPC product.
Figure 3: Close-up view of the finger area
Based on research and analysis, two technical approaches for producing single-sided perforated boards, as shown in Figure 4, prove feasible.
We will establish the manufacturing capabilities for perforated board products and verify the feasibility of the processes:
(1) PI windowing first + pure adhesive + copper foil process;
(2) Single-sided FCCL etching followed by PI windowing.
Figure 4: Manufacturing Process for Single-Sided Perforated Flexible Circuit Boards
Testing Methods
The testing methods are shown in Table 1.
Appearance-related inspections are conducted using a microscope, while dimensional inspections are performed using a coordinate measuring machine (CMM) with a sampling rate of 20%.
| Category | Item | Test Method | Inspection Frequency | Standard Requirement |
|---|---|---|---|---|
| Step Board | Key Area Warpage | Final Inspection | 100% Inspection | No warpage in key areas |
| Dimensional Characteristics | Microscope | 20% Sampling per Batch | According to drawing standard | |
| Hollow Board | Appearance | Final Inspection | 100% Inspection | No warpage in key areas |
| Hollow Position Accuracy & Dimensional Accuracy | CMM (Coordinate Measuring Machine) | 20% Sampling per Batch | CPK ≥ 1.33 |
Table 1: Experimental Testing Methods
Results and Analysis of the 3L Step Plate Experiment
The processing results for the step regions of the samples are shown in Table 2.
For the screen-printed peelable adhesive scheme, the acceptance yield for critical areas was 83.16%, with a process yield loss of 4%.
For the peel-off + carrier film application scheme, the process yield for critical areas was 97.44%, the acceptance yield was 94.23%, and the process yield loss was 3% (NG: Non-conforming products, Y: Conforming products).
| Category | Process Stage | Sample Quantity (pcs) | Qualified Quantity (pcs) | Yield Rate |
|---|---|---|---|---|
| Screen-Printed Peelable Adhesive | After Film Removal | 297 | 259 | 87.21% |
| Packaging | 297 | 247 | 83.16% | |
| Cover Film + Carrier Film | After Cover Removal | 312 | 304 | 97.44% |
| Packaging | 312 | 294 | 94.23% |
Table 2: Sample Processing Results
Quality Inspection of Peelable Adhesive Structure
Quality inspections verify the peelable resin thickness and resin overflow clearance for the screen-printed peelable resin solution.
Table 3 shows the results. The theoretical requirement for the support structure thickness is 71 μm, while the actual thickness ranges from 43.7 to 54.8 μm.
A peelable resin thickness of <71 μm meets the project requirements (sample size: 25).
The theoretical clearance distance is 0.2 mm, and the actual clearance range is 27.646–182.296 μm, meeting the design requirement of 0.2 mm clearance (sample size: 25).
Table 3 Inspection of Peelable Adhesive Thickness and Adhesive Free Area
Process Optimization and Final Performance Evaluation
The peelable adhesive support structure has a shorter coverage path, requiring a 0.2 mm adhesive exclusion zone on each side, resulting in incomplete coverage of the support structure.
As shown in Figure 5, high residual stresses concentrate in the exclusion zone, increasing the likelihood of wrinkling in critical areas.
The peel-off method has a longer coverage path than the peelable adhesive method and provides broader coverage of the support structure.
Figure 5: Analysis of the Low Pass Rate for Screen-Printed Peelable Adhesive
Process Optimization and Final Yield Improvement
Based on the sample processing, the following conclusions can be drawn:
- The screen-printed peelable adhesive solution exhibited incomplete film removal.
- After optimizing the film removal parameters for this solution, we reduced the removal speed by half.
- adjusted the thickness of the screen-printed peelable adhesive to 35 μm (single-pass screen printing);
- The laser depth-controlled milling parameters for Solution B were finalized, and single-axis machining was adopted.
- After final inspection and washing, wrinkles appeared in critical areas, causing yield rates for Scheme A and Scheme B to drop by 4% and 3%, respectively.
- Eliminating the final product washing step improved this.
Table 4 shows the results of small-batch processing.
The acceptance yield for critical areas in the screen-printed peelable adhesive scheme was 83.98%.
Changing the peelable adhesive thickness did not significantly improve project quality.
The acceptance yield for critical areas in the “peel-off + carrier film application” scheme was 95.83%, demonstrating a higher processing yield.
All process quality data for the samples, including board thickness and critical dimensions, met specifications.
Table 4 Results of Small Batch Processing
Test Results and Analysis of Single-Sided Open-Patterning Boards
The design includes a 50 μm thick copper layer and a 50 μm thick PI layer, following a process route that opens windows in the PI before patterning.
A die-cutting process creates the PI windows.
As shown in Figure 6, designing the PI windows first and then laminating created height differences.
These height differences cause poor laminate adhesion and lead to 100% line breaks in the open-patterning finger areas.
The manufacturing process involves 20 steps from PI windowing before patterning through to packaging.
Controlling the flatness at the step locations is challenging, resulting in the open-punch finger areas failing to meet both flatness and positional accuracy requirements.
Figure 6: Sample processing diagram
Optimized FCCL Etching and Post-Windowing Process
The process involves etching a single-sided FCCL board, followed by PI windowing, then applying a cover film and using an anti-plating dry film to isolate the finger areas.
As shown in Figure 7(a), the flatness of the cut-out fingers (FTY) was 64% (294/459), with a 100% test pass rate and a 0% breakage rate.
Indentations from the pre-treatment process during external electroplating caused the wrinkles.
Positioning accuracy of the cut-out fingers: As shown in Figure 7(b), the CPK is 5.1. The spacing, as shown in Figure 7(c), is 520 μm ± 20%, with no finger offset.
The process involves only 10 steps from pattern formation to PI windowing and packaging, and does not include lamination—a process step that most significantly affects step position flatness, making it a relatively superior design.
Figure 7 Sample Processing Diagram
Process Evaluation and Final Selection
The process route of “PI windowing first + pure adhesive + copper foil” is not viable; the finger breakage rate in the cutout areas is 100%, the manufacturing process is complex, and it is difficult to control the flatness of the cutout areas.
For the single-sided FCCL + PI windowing after the etching process, the CPK for finger positioning accuracy and spacing is 5.1, and there are no finger breaks.
Conclusion
This paper investigates the forming processes for stepped and perforated soft boards used in medical ultrasound systems.
The following manufacturing approaches prove feasible:
(1) 3L stepped soft board:
The screen-printed peelable adhesive scheme resulted in an 83.72% acceptance yield due to incomplete support structure coverage and a short coating process, resulting in a non-conformance (NG).
The “peel-off film removal + carrier film application” approach features a longer support structure coverage process and broader coverage area compared to the screen-printed peel-off film method, achieving a yield rate of 95.22% upon entry into inventory, which meets the project’s quality objectives.
Based on these results, we determine that the “peel-off film removal + carrier film application” approach (Scheme B) is suitable for producing stepped flexible boards.
We confirm that this approach demonstrates the capability for mass production.
(2) Single-sided open-patch flexible PCB:
Through a comparison of technical approaches, the single-sided FCCL + post-etched PI windowing process met product quality standards for both finger flatness and positional accuracy.
There were no broken lines in the finger open-patch areas, and the finger positional accuracy tolerance of ±20% corresponds to a CPK of 5.1. The single-sided FCCL + post-etched PI windowing process is capable of mass production.
The results resolve key issues. First, wrinkling in critical areas of 3L pure flexible stepped boards was addressed. Second, the manufacturing process for hollow FPC structures was validated.
Therefore, the study expands the technical scope of medical ultrasound FPC applications. It also builds a foundation for future development and production of these specialized structures.
Technical Achievements and Applications
Three practical outcomes came out of this work:
- A working process path for 3L stepped boards
- Verified routes for single-sided hollow boards
- Lower risk of wrinkling and finger cracking
For medical device teams, these outcomes matter during early design reviews. They also help when a project moves from prototype to volume production.
Conclusion
Specialized medical ultrasound FPCB structures are no longer optional. Miniaturization and high-density integration push probes toward a stepped flex circuit design and cutout layouts. This study shows two workable process routes for each. As a result, manufacturers can improve yield while protecting the most sensitive areas of the board.
FAQ
What is a medical ultrasound FPCB?
A medical ultrasound FPCB is a flexible printed circuit board built for ultrasound probe assemblies. It connects dense electronics inside a small housing. Typical designs rely on miniaturization, high wiring density, high interconnectivity, and efficient heat dissipation.
Why use a stepped flex circuit design in ultrasound probes?
A stepped flex circuit design places components on different levels instead of one flat plane. Therefore, it reduces interference between components and frees up layout space. It also improves spatial arrangement inside the probe housing.
What challenges do cutout flexible circuit medical boards face?
Cutout flexible circuit medical boards must keep the finger pattern flat inside the hollow area. If the process is wrong, fingers can crack or bend. Two routes help: PI windowing before copper lamination, or PI windowing after etching, each with anti-plating dry film protection on the finger areas.
