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102 DESIGN007 MAGAZINE I SEPTEMBER 2020 stiffener will have to be routed, or laser cut out of the main panel. Bonding with pressure-sensitive adhe- sive (PSA) tape is the most used attachment method available. The attachment is done with a PSA material, such as 3M/Tesa Tape, 3M467, or 3M9077. There are over 100 differ- ent types of PSA available from 3M alone. The specific PSA will depend on whether the flex- ible PCB will be subjected to a high-tempera- ture reflow cycle. The type of adhesive used will also depend on the configuration and/or location of the stiffeners. If a stiffener extends pass 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 place- ment. The flex and stiffener can both be accu- rately drilled with alignment holes, fitted over a small jig with alignment pins, and pressed together with PSA very accurately. Simply press the adhesive side down onto the flex and then peel off the poly-coated Kraft paper lining, aligning the stiffener into the pins and apply pressure to secure together. The PSA creates a very thin, aesthetic bond line caused by glue squish-out between the flex and the stiffener. The stiffeners can be laminated by hand or in an automation setup. The adhesive sheets are easy to die-cut or CNC rout, as well as laser cut. The PSA is easy to use, has a long life, and is an industry-leading solution for attaching stiffeners. 3M Adhesive Transfer Tape 467MP is the visually clearest of the 3M Transfer Tapes and offers a neat, pre- cise application with no mess or waste when applied to flat surfaces. It has excellent shear strength, as well as high temperature and chemical resistance, making it an ideal choice for stiffener attachment. The PSA tape features 3M high-performance acrylic adhesive 200 MP, an adhesive thickness of 2.0 mils (0.05 mm), and a poly-coated Kraft paper liner of 4.2 mils (0.11 mm). The 3M 9077 double-sided, PSA tape is a high-temperature adhesive and release liner system that is easy to use. This is the rec- ommended tape to be used for reflow sol- dering, as it survives short term exposure to opens in the copper traces if the bend radius is too small or the number of cycles is too large. A transition area from the hard mount, no flex stiffener to a fully flexible one will extend the life of a flex circuit. The 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 after the panel is laser routed or die-cut, using 3M double-sided tape, epoxy glue, or a lamina- tion process using a glue sheet prepreg. The FR-4 stiffeners will be routed out of the desired thickness of FR-4. The adhesive glue sheet is applied to the flex and the two parts aligned, using etch marks or silkscreen marks, and pressed together. The double-sided acrylic adhesive tape will securely attach the two parts together. This method also allows for the stiffeners to overhang the flex area and provide mounting holes or points (Figure 1). In addition to the pressure-sensitive tape, the parts can be thermally bonded with heat and pressure in a lamination press as a large panel. The 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 and gluing 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 that do not extend past the flex circuit. The flex final circuit and A transition area from the hard mount, no flex stiffener to a fully flexible one will extend the life of a flex circuit.