Flex PCBs and rigid-flex PCBs offer many useful advantages over rigid boards, such as in devices like wearables and mil-aero. The flex section is built from polyimide-based materials and adhesives, as well as copper foils, which build up a layer stack that can support reflow soldering. Assembly on flex materials generally follows similar procedures as standard rigid boards with two important exceptions: the need for fixation and pre-baking.
Flex PCB materials can absorb moisture that interferes with high-volume assembly, specifically during reflow soldering. There is also the chances of deformation during the reflow cycle, where the board is slowly heated up to a required assembly temperature. Some simple pre-assembly steps can help ensure maximum quality of the assembled flex PCBA, and the same steps could be necessary during rework.
Flex Reflow and Rework Procedures
There are two major procedures that can help ensure flex PCBAs are assembled and reflowed with highest possible quality and accuracy:
- Fixation, where the bare board is mounted in some fixture
- Pre-baking, where the board is baked at low temperature to remove moisture
These should be performed before reflow assembly to ensure the board does not have problems with tacky no-clean flux residue, problems with component shifting during reflow, or excess vibration as the board enters the reflow oven. The same ideas apply to rework, which could be done by hand or in an automated line.
The term “fixation” generally refers to fabrication of a fixture for a flex or rigid-flex PCB. The fixture will hold the flex section in place so that it does not deform during the pre-soak and reflow period of the soldering process. Holding the flex section steady could involve mechanical clips, placement of pins into through-holes, or taking advantage of mounting holes in the PCB. This will also help hold the components in place and prevent shifting during the pick-and-place operation.
Flex SMT soldering fixture. [Image credit: AGI Corporation]
Fixation can be a bit of a double-edge sword because it may fail to prevent vibration. If the flex section is made too taut during fixation, any vibrations in the reflow line could telegraph onto the flex ribbon. If the vibration is extreme, then placed components could fall off the flex section and the rigid section as the board enters reflow. This is another area where some experience is required to determine the optimal fixation force needed for a flex PCB.
The pre-bake procedure with a flex PCB should be low and slow. There are two reasons for this. First, the energy required for desorption of water from flex and rigid materials is not extreme compared to some solids, so extremely high temperatures are not needed. Second, the coverlay in flex buildups is acrylic and has low glass transition temperature (Tg). Due to the low Tg value of these materials, an excessive temperature in the flex section could lead to delamination of the coverlay from the underlying polyimide and copper.
The required bakeout time and temperature is typically on the order of hours and should be done just below the boiling point of water. For example, one recipe could be ~150 °C for several hours prior to reflow. If there is a problem with moisture desorption leading to weak solder or tacky flux residue, then some experimentation may be needed to determine the best pre-bake time for the bare flex/rigid-flex PCB.
Do Things Change With Rework?
Whether procedures need to change depends on the amount of time that has passed since the initial soldering process and the time rework is being performed. In a typical case where a defect is identified on the factory floor, a board will generally be removed from the assembly line immediately and will be sent for rework. There will basically be no time for appreciable moisture absorption since the initial pre-pake.
If rework is being performed immediately, then normal soldering/desoldering procedures can be performed, or the design can be sent back into processing for another reflow pass in extreme cases. However, not all cases of rework begin from a defect found in the factory. Instead, rework might be requested by a customer after testing and inspection.
In this case, when the board has already left the facility, and the end user requests rework, they will have to ship the boards back to the assembler for desoldering and rework. Just to be safe, the assembler should assume that some level of moisture uptake has occurred between assembly and the rework request. If the rework request requires hot air soldering, it is best to perform another brief bake cycle before starting rework.
Some flex rework may not require complete baking of the PCB.
For hand soldering, pre-bake is less important because heat is being applied directly to the conductive solder pads. There is not really any reason to bake an entire set of materials to encourage moisture desorption when the flex materials will not be heated during rework and re-soldering.
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