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Flex provides several advantages to cables. Flex’s higher upfront cost will dramatically reduce cost in the long run. Cables can take several weeks to go through production, while flex PCBs can be completed in several days. In short, using flex eliminates or completely decreases cable cost, reduces assembly time, and produces a lighter end-product.
Small form factor products which have constrained space and cannot accommodate connectors are ideals for flex boards. These applications include mobile phones, watches, laptops and their screens.
The real cost driver is the number of laminations. Vias should never be in an area that is dynamic. You don’t have to use stiffeners, but we do not suggest it in the flex area. Make sure vias stay out of the bending area as well.
We can do sequential lamination, from 1 to 2, 2 to 3, 3 to 4, 5 to 4, and from 6 to 5.
We will use sequential lamination for tightly spaced HDI boards.
Class 2 products follow a very basic manufacturing process. However, the class 3 manufacturing process has various checkpoints. Each and every class 3 board should undergo cross-sectional analysis. The cost of a class 3 circuit board could be 25% more than the class 2 build. The cost also depends on the manufacturers.
No-flow prepreg is the preferred bonding material for joining flex and rigid materials. This is most commonly found in standard FR-4 or Polyimide.
No flow prepreg is the preferred bonding material. Available in either standard FR-4 or Polyimide.
We use Dupont material to build circuit boards for medtech applications that come in contact with patients.
The flexing ability of a flex board mainly depends on the thickness of the copper layer and the substrate. Flexibility decreases as the thickness of the circuit increases. Cross hatched planes can be used to improve flexibility. This also is an ideal option for controlled impedance designs.
The cracking of vias during thermal cycling can occur due to high CTE contrast at Z direction between resin and copper. This can be mitigated by choosing matierals with similar CTE values.
Some flex materials, like Kapton, do not drill as well as regular materials. 10 mils and above is preferred for minimum size, only in the flex part. Rigid-flex specs are similar to rigid board specs. Pad size, plating process for flex is barrel-only (hole wall).
Flex material is hydroscopic (it absorbs moisture). Blistering comes from moisture trying to get out, and can cause thermal defects, like substrate blisters and barrel cracking during soldering. It also reduces dielectric breakdown voltage and expands the board. Pre-bake can stop blistering and eliminate moisture absorption.
The plane layers for the flex are the same as the would be for the rigid board. This board would be turned into a 6-layer board, as follows:
The total prepreg thickness must be greater than the coverlay thickness by at least 2 – 3 mils. If you have 2 mils of coverlay, you would need 3 – 4 mils of prepreg.
The number of layers depends on the design requirements. 12 layers of flex is not unheard of; however, the higher the number of flex layers, the more difficult it is for the PCB to flex. Yes, you can have hashed ground planes on either side of the signal layers.
Place parts where the flex has a stiffener or has been rigidized.
8 mils by 8 mils
Rigid-flex can generally hold the same tolerance as rigid PCBs. If you are talking about mechanical dimensions, +/- 5 mils is an acceptable tolerance.
Yes, pressure-sensitive adhesive (PSA) can be applied to flex.
The rigid part of the board is also part of the array.
The bonding material (prepreg) is removed from the flex region, and then the rigid part is milled as part of the final route operation.
Yes, 1 mil Kapton material can handle 5kV.
This depends on how critical your design is. Flex can handle buried vias. If there is a combination of buried and blind vias, it is recommended to stagger the vias as flex boards bend during their operation.
Flex circuit boards can generally handle temperatures upto 350°F (177°C). If these circuits are exposed to higher temperature for a long time, the substrates start melting. It also depends on the type of material chosen to build your board.
Yes, copper and core thicknesses affect minimum bend radius.
We have advanced machines in our facility to perform the drilling process. These machines can drill vias with controlled depth as low as 1.5 mils.
The minimum trace spacing required in FPC with 1 ounce copper is 4 mils.
Having via-in-pads in a flex board completely depends on your design. For a two-sided board, the via can be drilled from the bottom side without disrupting the pad on the top layer. For a multilayer board, a dog bone architecture can be implemented. Here, the trace will be routed away from the pad towards the via. Via-in-pad is not recommended in the multilayer flex boards as it is required to planarize the copper surface after via filling. If planarization is performed on the flex materials, there is a risk of damaging the substrate.
Yes, this type of construction is called loose-leaf construction or air gap construction method. In this method, the flex layers are kept as several independent sets of no more than three layers per set. IPC 2223 document defines the standard for this type of structure. Air gap construction can overcome the mechanical and electrical reliability issues in higher layer count rigid-flex designs.
If you would like to bond a finished flex circuit to an external frame/structure, pressure sensitive adhesive (PSA) can be used. Please, email us for more information.
Rigid and 2-layer flex boards can be fabricated in 24 hours. The typical turnaround time for rigid-flex is generally 5 to 10 days depending on the requirements.
Silkscreen doesn’t affect the flex properties of the board.
We only use no flow prepregs to build hybrid (rigid-flex) circuit boards. The bonding of these materials comes with various constraints. One simple solution to address this issue is to select the materials with similar CTE values.
First, it is important for you to understand that the PCB stiffener is not an integrated part of the electrical circuit board design. It exists just to offer mechanical support. We call a stiffener for when you need it. Here are some enlisted reasons to call for stiffeners:
Basically, you use a stiffener when you require a rigid area in your flex circuit, maybe to protect components or connectors attached there. This will not let the circuit bend and protect the integrity of the part’s solder joint.
The standard stiffener thickness varies from 0.002′′ to 0.059′′. The FR-4 stiffeners can be 0.008″ to 0.059″ thick while the Kapton stiffener thickness ranges from 0.002″ to 0.010″. The more support the PCB stiffener provides, the thicker it is. Each design requires a distinct thickness.
Flex is more difficult to assemble than rigid boards, because it is not as sturdy to assemble. We often create backing or fixtures to add additional reliability before beginning the assembly process.
The flex layer is processed first. This makes up one of the layers of the rigid multilayer. Plated through-holes are drilled after lamination.
Fiducials and tooling holes need to be on the rigid section or on the array rails.
Yes, components can be placed on the flex substrates. PCB stiffeners are placed underneath the component to provide mechanical support. This prevents the damage of solder joints when the substrate bends. If the board doesn’t bend, the components can be directly placed on the substrate. It should be noted that flex stack-up absorbs moisture during lamination. To avoid this, the assembly process should begin as soon as possible after you build your flex board. Scaling issues might occur when manufacturing flex circuit boards. The assembly machines should be smart enough to understand the dimensional variations and place the components accurately.
WLCSP can be soldered on flex circuits but these packages are not recommended over bending areas.
Yes, copper bumps can be placed on the pad. However, these bumps can not be placed on the circuitry. We sometimes make use of this technique to overcome the violations during pad plating.
Since we do both assembly and fabrication, WLCSP with 0.4mm pitch can be assembled onto FPC.
Leave it to the vendor. However, the engineer should supply array dimensions.
Yes. Making the S curve helps not only with flexibility but also reduces the likelihood of the traces cracking. Placing traces opposite also helps.