Fabrication Technology for a Type of FR4 Semi-Flexible PCB

Under the driving of portable electronics products' miniaturization development trend, flex-rigid PCB (printed circuit board) has received its wide range of applications. Plus, PCB industry has been facing increasingly harsh competition, which leads to the most emphasis on flex-rigid PCB fabrication technology progress. Due to complex procedure of flex-rigid PCB fabrication, some PCB manufacturers specializing in rigid PCB fabrication only fail to meet the demands of flexible PCB fabrication. FR4 semi-flexible PCB that will be introduced in this article is a type of circuit boards featuring flexibility, capability to be 3D assembled, and bendability.

Properties of FR4 Semi-Flexible PCB

• FR4 semi-flex PCB features flexibility, capable of 3D assembly, and is capable of changing its shape according to space limit.
• FR4 semi-flex PCB is bendable without affecting signal transmission.
• Based on concerning product's design, assembly plant labor or errors can be reduced with product's life span improved.
• Product volume will be shrinked, weight dramatically reduced, functions increased and cost decreased.
• FR4 semi-flex PCB shares simple fabrication procedure, compatible with rigid PCB manufacturers' current manufacturing capabilities.

FR4 Semi-Flexible PCB Fabrication Procedure

FR4 mid-Tg material containing filler is applied as substrate material to fabricate a 6-layer rigid semi-flexible PCB that is manufactured by mechanically manufacturing a milling groove. The remaining thickness should be controlled to maintain in a range 0.25mm±0.025mm. Moreover, FR4 semi-flex PCB has to be capable of being bent at 90° for more than 10 times with no cracks generated.


Fabrication procedure of FR4 semi-flex PCB includes the following phases: material cutting, dry film coating, AOI (automated optical inspection), brownization, lamination, X-ray inspection, holes drilling, electroplating, graphics conversion, etching, silkscreen printing, exposure and development, surface finish, depth-controlled milling, electrical test, FQC (final quality control), packaging.

Primary Issues and Their Solutions during FR4 Semi-Flexible PCB Fabrication Procedure

The leading issue taking place to FR4 semi-flex PCB lies in how to control accuracy and tolerance of depth-controlling milling. Due to effect of material structure and characteristics, rigid PCBs have to feature sufficient flexibility leading to excellent performance after 3D assembly without resin cracks or oil peeling that might be regarded as potential quality risks. Therefore, during depth-controlling milling procedure, board thickness uniformity, prepreg thickness, resin content and milling tolerance should be fully considered and the optimal remaining thickness range should be defined.


• Depth Controlled Milling Test A

Remaining thickness milling is carried out through Mapping method and is achieved respectively in accordance with remaining thickness of 0.25mm, 0.275mm and 0.3mm, after which board will be tested in terms of 90° bending. The remaining thickness of both 0.275mm and 0.3mm fails to meet the demand. Based on microsection analysis, the leading cause of board's failure to be flexible lies in the damage of glass fiber bundle. When the remaining thickness reaches 0.283mm, glass fiber has suffered from damage. Therefore, depth milling has to be implemented with board thickness, glass fiber thickness and dielectric situation considered. Since the thickness between solder mask oil surface and L2-plane copper is in the range from 0.188mm to 0.213mm, reliability requirement will fail to be met when remaining thickness is more than 0.275mm.


• Depth Controlled Milling Test B

Based on the above test and microsection analysis, dielectric thickness of copper between solder mask and L2 falls in the range from 0.188mm to 0.213mm and 90° bending fails to be done when remaining thickness exceeds 0.283mm. Thus, mechanical manufacturing can be carried out when remaining thickness is controlled within the tolerance range of 0.245mm±0.213mm. Because panels are relatively large in their size (400mmx450mm), they fail to be able to completely match with machine due to board thickness and warpage during remaining thickness maintaining in Mapping method. That'll directly reduce the uniformity of remaining thickness.


• Depth Controlled Milling Test C

Size shrinking brings forward effect to board warpage and machine uniformity. Panel first prototyped and depth controlled milling is carried out in accordance with set size of 6.3"x10.5". Then, machine uniformity will be measured by Mapping point measuring with vertical and horizontal interval of 20mm.


Based on FR4 6-layer semi-flexible PCB fabrication, a special fabrication method is applied and developed to mechanically control depth and maintain the remaining thickness. As a result, manufacturing procedure gets simplified; other procedures conform to ordinary parameters; depth controlling remaining thickness tolerance has been maintained to be within the range of ±20μm.

PCBCart Can Manufacture Semi-Flexible PCBs

PCBCart has rich experience to manufacture semi-flexible printed circuit boards. If you have similar requirements, please do not hesitate to reach us to discuss your project! We will provide practical and cost-effective solution asap.

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