Multilayer interconnections need electrical paths that remain stable through stress, vibration, and temperature changes. In aerospace, medical, and defense electronics, that reliability is a primary design constraint. Every connection must support consistent performance even as operating conditions push the assembly toward its limits. Via integrity is central to that reliability. The plated vertical pathways in a through-hole PCB must preserve conductivity and mechanical strength across thermal cycling and environmental loads.
All Flex Solutions has the flexible circuit manufacturing expertise to support high-reliability electronics where connection quality matters more than complexity. Keep reading to learn how to design for real-world stress while meeting strict quality standards
What are Vias and Through Holes in Flexible Circuits?
In through-hole PCBs, vias are plated holes that electrically connect conductive layers within a flexible circuit. While the terms via and through hole are sometimes used interchangeably, they do not always mean the same thing. All through holes are vias, but not all vias are through holes.
Types of Vias
- Through hole: Extends from the top layer to the bottom layer and is drilled after lamination. Common in double-sided circuits.
- Blind via: Connects an outer layer to one or more inner layers and requires controlled depth drilling.
- Buried via: Connects only inner layers and is drilled and plated before final lamination.
How is Via Integrity Tested and Verified?
Via reliability for through-hole PCB is verified through IPC-defined inspection criteria and destructive microsection analysis using test coupons placed on the panel border.
Microsection Testing Process
- A coupon containing representative vias is removed from the panel
- The via is cut in half and potted in epoxy
- Coupons often undergo thermal stress conditioning
- The plating structure is examined under magnification
- Results are compared to IPC6013 class requirements
Critical Inspection Areas
Pad and Annular Ring
The pad is the copper area that surrounds the hole. The minimum copper encirclement around the hole is known as the annular ring. If the hole does not maintain full copper capture, a breakout condition occurs.
IPC6013 defines allowable breakout and encirclement requirements by performance class, with tighter requirements for higher reliability classes.
Side Wall Plating
The side wall is the plated copper lining inside the hole. IPC6013 defines both minimum and average plating thickness requirements. Void allowances vary by class, with Class 3 circuits allowing no voids and requiring greater plating thickness.
Knee Transition Area
The knee is the radiused transition between the pad and the side wall. This area is particularly susceptible to cracking or adhesion failure. IPC requirements for the knee are more stringent than for the side wall, with zero void allowances in higher reliability classifications.
What Design Factors Impact Via Reliability in Flexible Circuits?
Flexible circuits are subjected to bending, twisting, and thermal exposure that can stress vias and surrounding materials. Improper layout decisions can compromise plating integrity, particularly in the pad and knee regions.
Key design considerations include:
- Maintaining sufficient annular ring capture
- Following a bend radius guideline of 10 times material thickness for dynamic flexing
- Keeping vias and solder joints away from flexible areas of the PCB or transition points
- Recognizing that flexible adhesive systems are softer than rigid PCB materials
- Avoiding stress concentrations near stiffener edges
- Using anchor spurs or tie downs where appropriate
- Apply strain relief beading along stiffeners in dynamic flex applications
Partner With All Flex
Via integrity is a long-term performance driver in demanding electronics. Following IPC-established design and inspection practices supports compliance and reduces field reliability risk in through-hole PCB architecture.
By collaborating with All Flex Solutions, you gain access to design feedback, material expertise, and testing practices aligned with demanding operating environments. Learn more about our solutions for flexible circuits.