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3 Important Elements You Don't Know about Buried and Blind Via in HDI Flex-rigid PCBs

Since its first use in highly reliable military equipment in 1980s, flex-rigid PCBs have been widely used in high-tech fields. Up to now flex-rigid boards have become one of the research hotspots in the PCB industry. Combining the supporting function contributed by rigid boards and the functions of high density and flexibility contributed by flex boards, flex-rigid boards are capable of completing the 3D assembly under different assembly conditions, meeting the requirements of lightness, thinness and small size of electronic products. Therefore, flex-rigid boards enjoy a wide application field.

Most flex-rigid boards have buried and blind via. When choosing the type of PCBs, there are lots of opportunities for you to pick up flex-rigid boards with buried/blind via, so it's necessary to know about them before quoting.

Advantages and Benefits of Flex-Rigid Boards

Nowadays, flex-rigid PCBs are widely used in portables, medical and military applications. Among all types of PCBs, flex-rigid PCBs have the strongest resistance against severe application environment, based on which their application fields will become wider. Because of its flexible feature, space size and system weight can be minimized by applying 3D.

In terms of the flex-rigid board design, it has been main trends that blind/buried via and High Density Interconnect (HDI) is required on flex-rigid boards. Because of the large number of types of flex-rigid PCBs, the following content will be displayed with 6-layer asymmetrical flex-rigid board as an example.

Delicate Material Preparation

The first layer of 6-layer asymmetrical flex-rigid board is flex board while the rest five layers of it are rigid boards. Line width spacing is 0.1mm and there are lots of blind/buried vias in rigid and flex-rigid area. The basic material is shown as Table 1:

Layer 3 and 4 Layer 2, 5 and 6 copper foil Layer 1 (flex)
FR-4 double-side copper inner layer Copper foil (one-side browning) Calendaring PI single-side copper clad plate

Because of the easy occurrence of delamination of flex-rigid boards, acrylic acid adhesive should be picked up as pre-preg of flex-rigid associative part in order to meet the viscidity requirement. For rigid layers, no flow bonding pre-preg is picked up. Table 2 shows the features of no flow bonding pre-preg.

Tested Items Unit Operation Condition Performance Index
Standard Value Protection Value
Resin Flow Amount %
TPC TM650 <3.0
Glass Transition Temperature °C DSC
CTE x axis
CTE y axis
CTE z axis
10-6/°C Ambient to Tg 15
z axis Expansion % 5°C-260°C 4.5 <4.0
Inflammability UL94
Solder Float (260°C) s A >180 >120
Peel Strength kgf/cm A 1.4-1.6 >1.43
Flexural Strength kgf/mm2 A 40-50 <32.7
Water Absorption % A 0.01-0.14 <0.20

Blind/Buried Via Technique

• Lamination method

There are two types of lamination methods: one-step lamination and step-by-step lamination. One-step lamination refers to the process of laminating all the inner layers for one time. Short PCB manufacturing time and low cost is an advantage of this method. However, it's difficult to position the cover player in the process of lamination and lamination defect, delamination and inner layer deformation can't be found until PCB etching. On the contrary, step-by-step lamination refers to the respective flex layer lamination and rigid layer lamination in which it reduces the difficulty of positioning cover layer and graph offset in inner layer and lamination defect can be discovered in time, making the best of the characteristics of rigid and flex board material. However, compared with the one-step lamination, step-by-step lamination requires more operating procedures, time consumption and assistant material with cost increased.

• Material

For flex-rigid boards with blind/buried via, step-by-step lamination is advised to be used in order to ensure the quality of blind via and high alignment accuracy. Inner layer lamination goes first and then does the inner-outer layer lamination. Silicone rubber as lamination material and PET release film as mold cleaner are used in both of the laminations.

• Drill technique

NC drilling and laser drilling are required for two times respectively on this type of 6-layer asymmetrical flex-rigid board and UV drilling is used on blind via drilling. Since UV drilling is an advanced technology with relatively complicated operation, an extra reasonable cost is usually required by PCB houses.

• PLASMA cleaning

Plasma cleaning is used to eliminate the dirt on via walls of flex-rigid boards. Plasma cleaning follows the process that plasmas with high active state generate gas-solid reaction together with acrylic acid, polyimide, epoxy and glass fiber. Then the generated gas and plasmas without being reacted will be eliminated by air pumps. This is a complicated physical-chemical reaction. In a word, there's something you should know about buried/blind via in HDI flex-rigid PCBs before quoting because all these things are closely connected with your cost, time consumption and product performance.

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