The Must-Knows for Flex PCB Fab

What are the advantages of using flex versus cables?

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.

What are the preferred hole and pad sizes for flexible PCBs?

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).

What are the best flex prepreg materials?

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.

What are the alternative prepreg materials for joining flex and rigid materials?

No flow prepreg is the preferred bonding material. Available in either standard FR-4 or Polyimide.

What does a real stack-up look like? (Not four signal layers, plane layers needed)

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:

Do you recommend thicker prepreg layers to join with the coverlays?

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.

Are there any design rules for placing parts on the flex circuit?

Place parts where the flex has a stiffener or has been rigidized.

Are there recommended dimensions for the hatch pattern?

8 mils by 8 mils

What is the most cost-effective type of via for HDI rigid-flex designs, stacked microvia or blind/buried vias?

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.

What is the purpose of stiffeners?

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:

-To strengthen any specific area of the board.

-To maintain proper thickness in the flex circuit.

-To support PCB components and connectors.

-To constrain the flexible parts to areas where they should be.

-To facilitate better handling of a thin flimsy circuit board.

-To keep certain areas of flexible circuits flat and stable.

-To meet ZIF connector specifications.

-To increase the bend radius of the circuit at the intersection of rigid and flex part. This will avoid the stress on flex part during multiple bending operations.

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.

Continue reading our guide to PCB stiffeners.

What is the temperature that the rigid flex material usually supports?

Temperature guidelines: Kapton has a high temperature rating but is limited by adhesive.

Copper foil: adhesive or adhesiveless material.

Lead-free: not flame-retardant, has less outgassing (used in the military).

Do S curves in the flex region help with flexibility?

Yes. Making the S curve helps not only with flexibility but also reduces the likelihood of the traces cracking. Placing traces opposite also helps.

Where are dimension tolerances specified?

Rigid-flex can generally can hold the same tolerance as, rigid PCBs. If you are talking about mechanical dimensions, +/- 5 mils is an acceptable tolerance.

How many layers can your flex region have? Can you have hashed ground planes on either side of the signal layers for noise purposes?

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.

Is it possible to create a flex section with exposed adhesive so the section can be attached to an interior surface of the enclosure?

Yes, pressure-sensitive adhesive (PSA) can be applied to flex.

How do you add a fully rigid panel around the outside of the rigid-flex design to aid in automated processing?

The rigid part of the board is also part of the array.

What is the difference between assembling flex and rigid boards?

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.

What is the order of assembly? Are through-holes/vias in the rigid section drilled after the layers are bonded together?

The flex layer is processed first. This makes up one of the layers of the rigid multilayer. Plated through-holes are drilled after lamination.

Do we need tooling holes and fiducials on each rigid section for component assembly?

Fiducials and tooling holes need to be on the rigid section or on the array rails.

Is it best to panelize a flex board, or leave that to the vendor?

Leave it to the vendor. However, the engineer should supply array dimensions.

When manufacturing rigid-flex, how is the rigid part removed around the flex board?

The bonding material (prepreg) is removed from the flex region, and then the rigid part is milled as part of the final route operation.

How to avoid blistering?

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.