|Ask the Experts|
May 4, 2020
What Are Realistic Guidelines for PCB Flexing?
IPC 600 indicates 0.75% or less bow or twist for PCB assemblies with surface mount components. 0.75% is practically no flex at all and very difficult to achieve. What are the experts views are on board flexing?
|Expert Panel Responses|
Flexing is a very different concept from Bow and Twist.
Bow and twist is essentially the flatness of a board as received. When you place it on a flat surface, how much bow and twist do you see? Do the corners turn upward? Is the board distorted in anyway? This can be caused by material selection in the stack up as well as lamination processing and cool down.
The 0.75% requirement for bow and twist can be difficult to achieve. It requires the design stack up to be balanced from top to bottom and to make sure copper is evenly distributed throughout the board. From a processing standpoint, the manufacturer much be sure to have a lamination stack that keeps the board flat and follows proper cool down in the lamination cycle so that no stresses are induced during lamination. Warped boards can be a real problem to populate, especially if they do not have any flexibility to lay flat.
Flexing is the tendency of a board to deflect when a load is put on it. There is no requirement for flexing. The thicker the board,the less flexing you will typically see. Flexing is also dependent upon the amount of reinforcement in a board. For example, a pure flex circuit will be very flexible as it has no glass reinforcement.
Another variable can be if the PCB is a rigid flex. This will create regions for flex in the middle of a pallet. The bow and twist requirement is limited to individual rigid sections. However, having rigid flex circuits in a pallet may introduce additional difficulties for those who have not had experience populating rigid flex boards.
When flexing is a problem, it is necessary to use fixturing during SMT population to support the parts to ensure successful population. This is common practice for flex circuits and is often used on thin rigid boards.
In the end, many suppliers have learned how to manage board flexibility during population. It is something that usually requires fixtures when the board deflects.
Minco Products Inc
Flatness of the PCBs is critical not only for processing but from a reliability point of view. I do not consider this standard is impossible to achieve. If you are not getting this level of flatness than the processing becomes an issue starting with the SMT solder paste deposition, pick and place and reflow.
Due to specific design constraints, some of your boards will not be perfectly flat. However, keeping them flat through the assembly process is a must. Depending on the components populating the board, the flatness % can make the difference between a good functional board and a non-conforming one. And I am talking here about the processes involving dramatic temperature fluctuations (SMT reflow, wave soldering, selective soldering).
Fixtures/pallets can be used for these processes. At the end of the process I recommend a cool off time before removing the assembly from the pallet and move it to the next process.
Engineering and Operations Management
IPC A-600 applies only to unassembled PWBs. The reason for the reference to SMT boards is that raw PWBs that will have SMT components assembled on them need to be flatter than boards that only get through-hole components inserted. Now, today there are precious few boards that use through-hole only technology, so as a rule, unassembled PWBs need to meet the 0.75% criterion.
During assembly, bow and twist may increase, or in some cases they may actually decrease. What is acceptable is entirely dependent on the design,including the design of the circuit card assembly and the overall product design.
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