Design007 Magazine


Issue link:

Contents of this Issue


Page 87 of 103

88 DESIGN007 MAGAZINE I MARCH 2021 is too small or the number of flexing cycles is too large. A transition area from hard mount, no-flex stiffener to fully flexible will extend the life of a flex circuit. e viscoelasticity of many 3M adhesives and tapes improves resistance to vibration fatigue by imparting flexibility to a joint or bonded area. Many modern flex ZIP FFC connectors use a self-adhesive tape wrap or a solid mount with screws, which then slip into a connector sol- dered to the rigid PCB. Typically, flex stiffeners are added aer the panel is laser routed or die cut, using 3M dou- ble-sided tape, epoxy glue or a lamination pro- cess using a glue sheet prepreg. e FR-4 stiff- eners will be routed out of the desired thick- ness of FR-4. e adhesive glue sheet is applied to the flex and the two parts aligned, using etch marks or silkscreen marks, and pressed together. e double-sided acrylic adhesive tape will securely attach the two parts together. is method also allows for the stiffeners to overhang the flex area and provide mounting holes or points, as in Figure 1. In addition to pressure sensitive tape, the parts can be thermally bonded with heat and pressure in a lamination press as a large panel. e typical method is to thermally bond the stiffener to the circuit with the same flex- ible adhesive that is used to attach the cover lays. Using high heat and pressure, then glu- ing the parts together, this method will result in a very strong, permanent stiffener bonding. However, using lamination as a method to glue stiffeners is limited to larger panels with a large number of stiffeners, which do not extend past the flex circuit. e flex final circuit and stiff- ener will have to be routed or laser cut out of the main panel. Bonding with pressure sensitive, adhesive tape is the most used attachment method avail- able. e attachment is done with PSA (pres- sure sensitive adhesive) material such as 3M/ Tesa Tape, 3M 467MP or 3M 9077. ere are over a hundred different types of PSA available from 3M alone. e specific PSA will depend on whether the flexible PCB will be subjected to a high temperature reflow cycle. e type of adhesive used will also depend upon the con- figuration and/or location of the stiffeners. If a stiffener extends past the flex circuit side, in most cases, it will require a PSA attachment with a locational stiffener outline added to the copper etching data or silkscreen to allow for accurate placement. e flex and stiffener can both be accurately drilled with alignment holes, fitted over a small jig with alignment pins and pressed together with PSA very accu- rately. Simply press the adhesive side down onto the flex and then peel off the poly-coated kra paper lining, aligning the stiffener into the pins and applying pressure to secure together. e PSA creates a very thin, aesthetic bond line caused by glue squish-out between the flex and the stiffener. e stiffeners can be lami- nated by hand or in an automation setup. e adhesive sheets are easy to die-cut, and CNC rout, as well as laser-cut. e PSA is a simple, easy-to-use, long life, industry leading solu- tion for attaching stiffeners. 3M 467MP is the visually clearest of the 3M transfer tapes, and offers a neat, precise application with no mess or waste when applied to flat surfaces. It has excellent shear strength, as well as high tem- Figure 2: A 5-mil double-sided, thick cover coat used as a stiffener on a 1-mil very flexible circuit.

Articles in this issue

Archives of this issue

view archives of Design007 Magazine - Design007-Mar2021