Study On Manufacturing Technology And Process Of Semi-flex Printed Circuit Board

At present, the development tendency of electronic products’ miniaturization,multi-functionalization and high-reliability requires that the encapsulation technologyshould develop to three-dimensional encapsulation. Rigid-flex Printed Circuit Board(PCB), which incorporates the merits of rigid PCB and flexible PCB, can offersupportive function of rigid PCB and achieve local bending like flexible PCB. So it iswidely used in three-dimensional encapsulation of electronic products. As a type ofrigid-flex PCB, Semi-flex PCB is manufactured by ordinary bendable rigid PCBmaterials based on rigid multi-layer PCB technology. The flexible polyimide materialsare not need in Semi-flex PCB so that the cost of materials and manufacturing processcan be reduced. Semi-flex PCB which can offer better heat-resistant quality and morestable electrical performance is broadly applied in some electronic products which arenot used for permanent flexible operation but will be bent only a few times, e.g. forinstallation, rework and maintenance. In this study, Semi-flex PCB whose flexible areawas protected by polyimide cover-layer was manufactured by Opening-windowsMethod or Buried Material Method. Both were based on the manufacturing process ofordinary rigid multi-layer PCB and the special technological process was researchedmainly.In Opening-windows Method, widows in no-flow prepreg were shaped in thecorresponding location of flexible area in advance. Impending rigid part was peeled bydepth-controlled milling, exposing the flexible area. In this study, the180degree bendtest was carried out to evaluate the flexibility of different thickness of FR-4rigidmaterial. Mechanic milling was used to shape windows in no-flow prepreg in advanceand the machining parameters were optimized. The relationship between amplificationsizes of windows and length of prepreg resin flowing into flexible area in laminationprocess was studied and the optimal amplification size was obtained. The machiningaccuracy and feasibility of depth-controlled milling were evaluated. Reliability testsincluding Thermal Stress Test for5times, Thermal Shock Test for200cycles and SaltSpray Test for48hours were conducted to evaluate the reliability of products. In Buried Material Method, widows in high-flow prepreg were shaped in thecorresponding location of flexible area in advance. Buried material was used to protectflexible area through preventing resin from flowing into the flexible area. Rigid part waspeeled by depth-controlled milling and the buried material was taken away, thusexposing the flexible area. In this study, the preventing capability of resin for threedifferent buried materials including epoxy resin base material, Teflon (PTFE) andsilicon sheet were studied and the results showed that the silicon sheet was the mostappropriate one. Laser cutting was used to cut the silicon sheet into required size. Theinfluence of minification sizes of silicon sheet, amplification sizes of prepreg windowsand lamination heating rate on the resin flowing length was researched throughorthogonal experiments and the optimal process parameters were achieved. Verificationexperiments were conducted to validate the optimal process parameters. Reliability testsincluding Thermal Stress Test for5times, Thermal Shock Test for200cycles and SaltSpray Test for48hours were conducted to evaluate the reliability of products.Both Opening-windows Method and Buried Material Method had their ownadvantages and they could be chose according to different products’ requirements. Inthis study, Semi-flex PCB which was manufactured by these two methods could meetthe requirement about resin flowing length <1mm. The Semi-flex PCB could pass thereliability tests such as Thermal Stress Test, Thermal Shock Test and Salt Spray Test,showing good reliability performance.