Issue link: https://iconnect007.uberflip.com/i/1042925
50 FLEX007 MAGAZINE I OCTOBER 2018 needs to go in the same direction as the bend. Solid copper areas, such as ground planes, should be cross-hatched when possible. This will help make the circuit more flexible. Fabrication With the design complete, the next step is fab- rication. Flex circuits are more difficult to manu- facture than traditional rigid boards. Therefore, flex circuits require more manufacturing time, which results in the need for a longer lead time. There are several issues that lead to an increase in overall manufacturing time. First, flex circuits are built using a polyimide mate- rial that is thin, fragile, and difficult to handle. The drilling of the vias and the chemistry to plate the vias is different. In addition, a signifi- cant amount of hand labor is involved with the stiffener(s) and coverlay. The result is a mini- mum manufacturing time of three working days for a two-layer flex circuit. With a higher layer count, the manufacturing time can be upward of two to three weeks. Manufacturing rigid-flex circuits is very dif- ferent than rigid board manufacturing. The planning and CAM-ing of a rigid-flex can eas- ily take two to three days to perform before the board can be released to the manufacturing floor. This upfront engineering work is critical. There are many steps in the manufacturing process, and every step is essential to the suc- cessful manufacture of a rigid-flex board. One of the most important things to note is rigid- flex comes in many different stackups. It is rare to see multiple rigid-flex jobs on a manufactur- ing floor with similar stackups. With a rigid board, all six-layer boards are processed the same way. With rigid-flex, five jobs with six layers can be on the manufacturing floor, and all will be processed differently. For example: • Board #1: 6-layer board with 4-layer rigid and 2-layer flex • Board #2: 6-layer board with 3-layer rigid and 3-layer flex • Board #3: 6-layer board with 4-layer rigid and 2-layer flex with the two flex layers on different layers The other difficulty in rigid-flex manufactur- ing is the combination of working with two different types of materials. Rigid-flex is com- bining rigid material with flex circuit material. Each kind has different properties that make it tricky to work with; therefore, circuit board manufacturers rarely build rigid and flex cir- cuits in the same facility. The most common failure with rigid-flex circuits is attributed to the plating process. If the plating of the vias is not done properly, it will lead to voids, cracking, and delamination. The two differ- ent materials have different Z-axis expansion rates so that improper plating will expose poor quality. Assembly The last process in getting the flex circuit built is mounting all the components onto the flex circuits. For the most part, assembling components onto a flex board is very simi - lar to assembling components onto a rigid board. The three main things to keep in mind are moisture sensitivity, flatness, and handling. When it comes to flex circuits, moisture sen- sitivity is a real concern. Flex circuits are made from a polyimide base material that read- ily absorbs moisture over time. If moisture is entrapped in the flex circuits, it increases the chances of delamination during the assembly reflow process. It is a good idea to bake the flex circuits prior to the assembly operation to remove this moisture. As an assembler, it is a good practice to fol- low this baking procedure on all flex circuits because the environmental conditions the flex circuits were subject to before assembly are not known. Flex circuits built in California will be exposed to a different environment than ones made in the high humidity of Georgia, for example. A flex circuit that has been sitting in a stockroom for a period of time will surely absorb moisture. Without knowing these con- ditions, all assemblers should bake flex circuits before assembly. Green Circuits' standard pro- cedure is to bake all flex circuits for 6-8 hours at 150°C before assembly to ensure complete removal of moisture.