6-Layer HDI Rigid-flex PCB

Name: 6-Layer HDI Rigid-flex PCB
Material: FR408+PI
Layers: 6L(1+1+2+1+1)
Thickness: 0.6mm
Surface Finish: ENIG
Min Trace/Width: 4/4mil
Application: Automotive electronics

Mechanical bend reliability is a critical element in a successful flex or rigid-flex PCB design. One area that can have a significant impact is the transition line(s) that occurs from a rigid area, in a rigid-flex circuit design, or from a stiffened area, in a flex circuit with stiffeners design, to the flex layer(s) only areas.

This vertical linear change in the part’s construction thickness potentially creates a mechanical stress concentrator depending upon the bend requirements. If this situation occurs and is not addressed, it can cause the flex area circuits to be damaged when bent into the required shape.

This concern can be addressed in most designs by adding a strain relief preventing the flex layers from being bent beyond their physical capability. In this blog we will discuss why strain reliefs may be required, the materials used and design considerations that need to be applied.

Strain Relief

The purpose of a strain relief is to protect a part from stresses that can occur when it is bent or manipulated at a transition point within the design. The most common examples are wiring applications where a cable passes through an opening in an enclosure. If left unprotected the cable can be damaged or broken when bent or pulled against the hard edge of the enclosure opening. Strain reliefs exist in many formats from pass-through bushings, which protect against fraying at a hard edge, to over molds which force the cable to bend in a smooth arc preventing it from exceeding its bend capabilities and breaking or cracking.

Why Epoxy Strain Reliefs May Be Required

A similar situation, as with wiring applications, can occur where the flexible circuit layers extend out from within the rigid areas of a rigid-flex or where a component area stiffener ends in a flex design. This combined with a bend requirement that is in close proximity to the transition can cause the flex layers to bend sharply rather than form a smooth radius. A sharp crease will exceed the minimum bend capabilities of the flex layer construction. This will cause the copper circuits to deform, work harden and lose ductility. If the flex layers are then straightened and or bent again there is a very high likelihood of the copper circuits cracking and creating either completely open or intermittent open connections.

A second reason for a strain relief is specified in IPC 2223 Design Standard for Flexible Circuits Section 5.2.9. Some rigid-flex designs may result in a small amount of pre-preg, used to laminate the rigid and flex layer together, extruding out from within the rigid layers onto the surface of the flex layers. Once cured pre-preg forms a hard sharp and slightly ragged edge. If left unprotected and combined with a bend requirement close to the transition, can result in the flex layers being cut or torn when bent into position. While not all designs require strain reliefs it is very common to have them as a default requirement in high-reliability Class 3 designs.

Strain Relief Materials

A rigid-flex or flex circuit strain relief is created by adding a bead of a flexible material to the transition. This material, when cured, creates a tapered fillet. The bead is applied using either a manual or pneumatically assisted syringe. This allows a relatively precise bead to be deposited right along the vertical edge of the rigid area or stiffener. The tapered shape combined with the material’s flexibility encapsulates any exposed pre-preg and forces the flex layers to take a smooth bend arc.

There are a variety of materials that can be used as a strain relief. While RTVs and silicones can be used the most common in the industry is Henkel Loctite’s Eccobond 45/15 two-part epoxy system. This system allows it to be mixed in different ratios which result in either a rigid, semi-rigid, or flexible material when fully cured. Only the flexible formula mix ratio can be used. The additional benefit of Eccobond is that it can withstand reflow temps. This allows it to be applied to the bare circuit before assembly by the flex circuit manufacturer. All other materials must be applied after assembly by the assembly facility as they cannot withstand re-flow temps.

Design Considerations

There are a few design requirements for the application of strain reliefs. The first item is the height difference between the surface of the flex layers and the surface of the rigid areas in a rigid–flex PCB design. A minimum of 0.010” is recommended to allow for sufficient space to apply the strain relief. This is driven by the viscosity of the material. If the strain relief is applied too thick, it may extend above the surface of the rigid area and then prevent the solder paste stencil from laying flush and creating assembly difficulties.

The second item is the length of the flex section connecting between rigid sections. The typical resulting width of a strain relief bead is between 1 – 2mm. The properties and viscosity of the material cause the material to run out on the surface of the flex layers. The typical min. manufacturable flex length is 3mm. If the strain relief is applied to both ends of a 3 – 4 mm long flex section, there may be insufficient flex length not encapsulated by the strain relief to meet the design’s bend requirements. In the worst case, the flex area may be completed encapsulated by the strain reliefs.

Strain reliefs are typically applied to both sides of the flex layers at a specific transition location. In some unique applications, such as designs with unbalanced or offset flex layers in the construction, the strain relief can only be applied to one side of the flex layers due to insufficient height to the rigid area.

Summary

Flexible Epoxy Strain reliefs are an effective solution to many designs that ensure the mechanical bend reliability and prevent inadvertent damage to the flex circuit when installed in the assembly. Please feel free to contact QFPCB if you have any questions or require support in developing a rigid-flex or flex design to determine if it requires the addition of epoxy strain reliefs.

QFPCB has 15 years of experience in the field of Rigid-flex PCB production, choose QFPCB, you will get a reliable supplier in China. Contact us today for your Rigid-flex PCB solutions.

Flex And Rigid-Flex Circuit Board Terminology

Below are some common terms or abbreviations used in the flex and rigid-flex circuit board world. Some of these terms are common but some less common are worth a mention. The languages spoken across the world are changing, from the everyday talk as we knew it to the lingo. Talk-to-text, no matter where you live, we are seeing more shortcuts. To emoji and abbreviations used everywhere it is common to shortened text to make a point like, FPCB or RFPCBs to name a couple.

FPCB: flex printed circuit board is a PCB that bends, flexes, and can twist to fit into a product.
RFPCB: rigid-flex printed circuit board is a PCB that bends in a section or sections and has multiple rigid areas.
Loose Leaf: refers to several flex areas that are not bonded together at the flex portion.
Anchors or Tiedowns: are plated through holes within a flex area that are tear dropped for the stability of the flex areas, typically a mounting area.
Tear Drops: are circuity built-up areas at circuit-to-pad interface that form the shape of a teardrop.
Stiffener(s): are typically either polyimide or FR4 that are added to the flex area to aid in strengthening the flex for assembly of some sort.
Cross-hatching: is a form of non-solid ground plane that aids in flexibility of the ground a solid area will not allow the flex to bend.
PSA: is a pressure-sensitive adhesive and is applied to the flex or rigid-flex to allow bonding to the end product.
Transition Zone: the area where the rigid material and flex materials meet or where the flex exits the center of the rigid stack of material.

Dos And Don’ts Of Flex And Rigid-Flex PCB Data Sets

One of my favorite subjects regarding our printed circuit board manufacturing specialty is data. I love to look at data files. So, some of the dos and don’ts regarding the design of FPCB or RFPCB that should be considered early on in your design layout stage are critical to successful manufacturing in small or large volume. These rules will help you and your PCB supplier minimize fallout and the need for costly revision changes.

When it comes to flexible circuits of any type there are some additional parameters to consider. For starters, the shape of the part, where it will be used, the space that is needed to fit the part including length width and thickness after bending and flexing into the final shape. The outline(s), that is right more than one, are critical to engineering and productions success in building the part correctly. Don’t get me wrong in rigid circuit board manufacturing, the outline is very important as well but with flex and rigid-flex circuit boards the considerations are different.

First, there can be many outlines, the overall, all-inclusive outline, the flex portion, or portions outline and the stiffener(s) outline. All important to your fabricator for ease of process and labeling them accurately.

The stack up of the part for all flavors of this unique process is a critical piece. When possible, leave the stack up for the fabricator to determine what is best for the product and advise your fabricator if there is an item pushing your product to a non-balanced construction. In the rigid circuit board world, we push for balance uniform construction with balance copper foil. The same is true for the flexible circuit board world where it is recommended to have the flex exit the rigid area from the center of the balanced layup.

Materials are not always available; if the type of material is not critical then leave that up to your supplier as well.

How Do Flexible Circuits Flex?

On your final product the flexible circuit bend radius maximum is 10x the thickness of the flex portion. Why is this important to the circuitry? For layout of the areas to be bent there should be no right angles in the trace layout, in fact using a smooth curve radius is what is best for producing the part and for bending the flex. Bending flexible circuits with sharp corners will cause the circuit to break in these areas, so smooth out those to curves.

Regarding the traces, during layout stagger the traces from side to side on multilayers, staggering the traces will allow for the “thinness” to be maintained. Keeping the layers of copper tracks off each other from side to side will help keep the bend and thinness working together.

As for holes, vias specifically, use as few holes as possible in the design. Keep in mind a 0.010” hole will need 0.010” annular ring and that the flex process needs more annular ring than the rigid. A 0.010” hole will need 0.030” copper pad to allow for sufficient annular ring. For the larger mounting holes used to attach the flex to the end product, these large holes should also have 0.010” annular ring as well as teardrops to increase the pad to circuit integrity.

Rigid holes to flex bend area need to be carefully placed. All holed should be kept a minimum of 0.050” from the transition zone. Ground areas or polygons must be hatched, a solid plain pour will prohibit the parts bend so no solids.

All circuitry needs a coating of protection, whether it is a plating layer or a masking it is needed. The rigid-flex PCB may use both LPI, liquid photo imageable mask and what is known as a coverlay.

A coverlay is used on the flex areas, due to the bending of the part and the nature of coverlay and its flexibility. LPI has a much tighter control than coverlay does, however they both serve the same purpose to protect the copper from the elements, shorting or damage. Keeping in mind coverlay is routed and placed onto the surface and the openings must be much greater than the LPI mask. Allow for gang masking where mounting devices or components are being placed. Keep circuit to pad spacing at design rules that apply.

Always use a design for manufacturing (DFM) service to check and verify your data set for manufacturing. Finding issues upfront during the design and layout phase are time and cost savers. Our free DFM service is accessible on our website and available 24/7 to upload your data files. You will have a direct line to our engineering staff for a completed review, suggestions and correction needed prior to production.

When To Involve Your PCB Fabricator

Involving your PCB fabricator during design and layout stage is advantageous to ease of manufacturing. More often than not manufacturing is not considered at development which can lead to costly prototyping, re-spin of the data set and delays to market. With your circuit board fabricator involved early on to review and comment on the design and even perform a real time DFM it is always a win for both sides.

Summary

The main purpose of a flexible circuit board or rigid-flex circuit boards is to improve the integrity of the product while saving space or aiding in reducing the size of the final product. The cost can be up to 2 times that of a rigid circuit board or significantly more than a cable assembly, but the reliability from this product and the use of it is on the rise. Knowing how to properly design a rigid-flex PCB or flex PCB is key to your products success, involving your supplier and its professionals is the best, quickest way to market.

For personal assistance in addressing your designs and options with a thorough design review and input from an experienced supplier. Please feel free to contact QFPCB if you have any questions.

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