Well known, rigid-flex board consists of flexible materials, FR-4,adhesive, CVL and other composite systems. An obvious problem in applying HDI to rigid-flex board is that the processing conditions of laser blind holes are complicated and the reliability is difficult to guarantee due to the variety of rigid-flexible plywood materials and changeable media. The laser blind hole of rigid-flex plate can be divided into three categories according to the dielectric layer material where the blind hole is located, one is rigid dielectric layer with FR-4 as the main, the other is flexible material with PI as the main, and the third is laser blind hole through PI flexible material and mixed dielectric with PP or adhesive. the reliability of laser blind holes in single rigid dielectric layer or flexible dielectric material has been studied more in the industry , but there are few related studies on the reliability of laser blind holes in rigid flexible mixed dielectric layer materials. Through the design of a rigid-flex board, the laser blind hole is designed in the mixed dielectric (PI and PP), the process parameters of the laser blind hole are studied and compared, and the influence of the processing process on the laser blind hole in the mixed dielectric is studied.

Effect of Laser Drilling Method on Making Laser Blind Hole in Mixed dielectric

Reliable laser drilling is the premise of making qualified HDI blind holes, and the hole shape and aperture of the blind holes will seriously affect the hole metallization and reliability of the post-process. Generally, the blind hole aspect ratio, upper and lower aperture ratio, hole type and so on have strict requirements. There are two main processes in the industry to make HDI laser blind holes, CO2 laser drilling and UV laser drilling. The main principle of laser is to use infrared laser to make matter absorb a lot of energy and then heat up and melt, volatilize and burn. UV laser is to use ultraviolet laser to directly destroy the molecular chain (organic matter) or metal bond of matter and disperse it into atomic / molecular group.

The only difference between the HDI rigid PCB and the HDI rigid-flex PCB is the difference of the dielectric. In order to ensure the consistency of the laser drilling parameters, the conventional PP type laser drilling parameters are selected uniformly.PI/PP mixed dielectric and PI/ adhesive mixed dielectric test board were used for UV laser drilling and CO2 laser drilling, respectively. the data of hole type and hole foot / hole opening under different laser drilling conditions were collected with the ratio of hole foot / hole opening 0.6-1 in the industry and USL, respectively. The results are shown in figure 1 and figure 2.

We can see from figure 1 and figure 2 that under the same laser drilling condition, the hole type after laser drilling PI/ pure glue mixing medium shows the trend of “trapezoid” with upper narrow and lower width. the hole foot/hole ratio is much larger than 1, which can not meet the design requirements, while PI/PP laser blind hole type of mixing medium meets the requirements, and there is no obvious difference between the laser blind holes made by both UV and CO2 laser drilling. After analysis, it is concluded that the laser has different ability to excite different materials, and its ability to excite organic matter is much larger than that of inorganic matter. The medium composition of the flexible substrate is polyimide, while theerted trapezoid “. From the point of view of the pore shape, the laser erosion abilit pure adhesive only contains organic epoxy resin, while the PP contains inorganic glass fiber and organic epoxy resin. For the PI/ pure adhesive mixing medium, the excitation of organic matter during laser operation is particularly large, even through heat transfer (CO2 laser) or light transfer (UV laser) etching to the side wall of the laser, thus showing that the PI/ pure adhesive mixing medium has a large partial pore shape; PI/PP mixing medium, because the PP contains glass fiber which is difficult to corrode, absorbs more energy and prevents the blind hole foot from expanding further, From the point of view of the pore shape, the laser erosion ability is adhesive >PI>PP.

PI/ laser blind hole made in pure rubber mixing medium is not recommended because the laser hole type is easy to become an inverted trapezoid with upper narrow and lower width due to more laser etching under the action of adhesive, which is very unfavorable to the subsequent hole metallization, and its reliability is poor.  The blind hole can be made PI/PP the laser blind hole in the mixed medium with conventional PP type laser drilling parameters. the hole type aperture all meet the design requirements. considering the production efficiency requirements, it is recommended to use CO2 laser drilling for production.