The most important item is to Select the Material of Rigid-Flex PCB. There are some items for the material you can visit, but for much more you can join the Rigid-Flex PCB group to discuss much more.
As the saying goes, “if you want to be good at your work, you must first make use of your tools.” so it is very important to make full preparation when considering the design, layout, and fabrication process of a rigid-flex combination board, but it requires certain professional knowledge and understanding of the characteristics of the materials required.
Flexible Base Material Selection is a key item to select Material of Rigid-Flex PCB: Polyimide Flexible Base Material
We select DuPont’s (AP adhesive-free Series) polyimide flexible base material as the copper-clad material of the flexible board. Polyimide is a kind of material with good flexibility, excellent electrical performance, and heat resistance, but it has great hygroscopicity and poor alkali resistance.
The reason why the base material without the adhesive layer is selected, as the adhesive between the dielectric layer and copper foil is mostly acrylic acid, polyester, modified epoxy resin, and other materials.
Among which the modified epoxy resin adhesive has poor flexibility, the polyester adhesive has good flexibility but poor heat resistance, while acrylic acid adhesive has satisfactory heat resistance, dielectric performance, and flexibility, but it needs to be considered the glass transition temperature (TG) and pressing temperature are relatively high (around 185 ℃).
Then we currently use Japanese (epoxy resin series) base materials and adhesives to match the rigid-flex PCBs.
Rigid Plate Material Selection-Select Material of Rigid-Flex PCB
There are also certain requirements for the selection of rigid plates. The choose the epoxy board with low cost, because the surface is too smooth to be adhered to, and then choose fr-4.g200 and other base materials with a certain thickness to etch the copper. However, due to the difference between fr-4.g200 core material and PI resin system, TG and CTE do not cooperate together, and the warpage of the rigid-flex combination part after heat shock is serious Therefore, we use rigid material of PI resin series, P95 base material, or it can be simply pressed together with p95 semi curing sheet. In this way, after the rigid and flexible are pressed together, the warpage deformation after heat shocking can be avoided. At present, there are many base material manufacturers who develop and produce some rigid plate materials especially for the combination of rigid-flex boards.
Adhesive Part between Flexible Material and Rigid Plate-Select Material of Rigid-Flex PCB
No flow adhesive is preferred for the adhesive part between the flex part and the rigid plate, the No Flow (low flow) prepreg is used for pressing, because its small glue fluidity is very helpful to the soft and hard transition area, and it will not cause rework or functional impact of the transition area due to glue overflow.
At present, many enterprises can provide prepreg raw materials that have been developed as this kind of prepreg sheet and many kinds can meet all structural requirements. In addition, for customers requirements for RoHS, If there are requirements for high Tg and impact, it is also necessary to pay attention to whether the characteristics of raw materials can meet the final
requirements, such as material thickness specification, dielectric constant, TG value, environmental protection requirements, etc