Flex PCB design guidelines

Flex PCB design guidelines


The purpose of this design guide is to enable you to design a highly reliable, flexible printed circuit optimized for manufacturability.

This guide provides technological data for choosing the appropriate materials, and recommendations for their correct design, while taking into account their integration criteria and constraints through assembly processes and car environment.

Below guildelines including these contents:

  • Flex pcb board standard stack up
  • Flexible pcb production process flow
  • Flexible circuit Design Guides and Rules

Flexible Circuit Board Types or Flex pcb Construction

A wide range of available LEDs put varying thermal demands onto the PCB substrate. Low-wattage (0.25W LEDs) and low-density applications are typically dealt with by using standard, single-sided FR-4 or CEM PCBs, where all the heat must be dissipated at the surface and the thermal performance is enhanced by using large copper lands (for heat spreading) and higher copper weights when required. The FR-4/CEM materials are very good thermal insulators and so obtain little or no benefit from a secondary heat sink and the operating temperature is directly influenced by the ambient temperature and although this does limit the use of this technology, it still represents a significant part of the LED market. It should be noted that there are some new FR-4/CEM style laminates that have been developed with a higher thermal conductivity, which allow the LEDs to benefit from secondary heat-sinking.

For mid-power (1.0W LEDs), moderate density applications, where the thermal requirements are beyond the capability of a standard, single-sided PCB, the next level of thermal performance comes from FR-4 PTH PCBs using thermal vias to enhance heat dissipation. The heat generated by the LED spreads across the pad and then down the plated via holes to a large copper area on the other side of the board, this heat can then be dissipated into a secondary heat sink. The holes around the LED pads do limit the potential LED density, and from our experience we find that holes placed further than 5 mm from the LED have a much reduced effect on the junction temperature. Obviously, the use of via-in-pad technology will allow for higher LED packing densities but this does create other assembly issues (and if this means using hole-filling, then any cost-savings for using FR-4 will be eroded); however, via-in-pad will improve the thermal performance when compared to having vias around the LED.

Single Side Flexible PCB

Double sided flexible circuit

Multilayer flexible PCB

Blind and buried via flexible circuit

Flexible circuit manufacturing process flow

Design Guides and Rules

1. Bonding Sheet Design (Folder area)

B/S design by drawing
  • If the border line type of NO ADHESIVE AREA part is vertical, it can cause SHORT or OPEN circuit problem.
For prevention of defect B/S design
  • We prefer to design the Bonding Sheet in the manner that gives a slope of 45 degrees to one of the border lines of LCD mounting area or TAIL part. (Exception: Single sided type may not be affected)

2. Pattern design of Folder Area

Pattern design of Folder Area
  • Purpose: To retain the maximum flexibility by staggering the pattern lines.
  • Method:
    1. Pattern lins on each layer will be staggered. (as much as possible)
    2. Pattern lines on 1st and 2nd layer are staggered. (Refer to diagram beside)
    3. Pattern lines on 3rd and 4th layer are staggered. (Refer to diagram beside)
    4. As result, pattern lines on each layer can be staggered with each other.
    5. This is to be considered for signal pattern line.
  • Reason: If the pattern of every Layer is located above the same line, it causes decreate of flexibility.

3. Silk Screen Specification

Silk Screen Specification
  • Purpose: avoid possible default by understanding silk screen production condition.
  • Method:
    1. Text Mark: Customer's mark, Symbol, Date code; Size 2mm
    2. Component Test Mark: Min. 0.7mm, Max. 1.5mm, We may shift the marking position depending on the situation. (After discussion with the customer)
    3. Insulation Line:
      1. Line for preventing short circuit between the lands;
      2. Line thickness: 0.15mm (standard);
      3. Distance between line & land: 0.2mm;
    4. Land Out-Line:
      1. Silk screening on the outer line of the land is of no use.
      2. Prefer to remove unless it is an insulation lin (After discussion with the customer).
    5. Alignment Line: Follow the customer's request.
    6. Space: Min. 0.2mm between the lines. If it is beyond specification, it will be shifted after approval from customer.

Silk Screen Tolerance

Silk Screen Tolerance
Item Dimension
A (Min. Width of Marking) Min. 0.15 mm
B (Min. Distance from Land) Min. 0.2 mm

4. Pattern line thickness & tolerance of raw material

Pattern line thickness & tolerance of raw material
Two Layer type L (Min. Line) S (Space-pattern / Pattern) A (Space-pattern / Border) R (Min. Radius Value)
1/2 oz 0.005 (±10%) 0.005 0.2 0.2
1 oz 0.075 (±10%) 0.075 0.2 0.2

5. Through hole / Pad (Inside)

Through hole / Pad (Inside)
(Unit:mm)
Mechanical CNC Laser N.C
A 0.10 0.10
B 0.40 0.30
C 0.10 0.10

Stiffener / Tape Area Tolerance

Stiffener / Tape Area Tolerance

6. Tear-drop design

Tear-drop design

7. Cover lay & Solder Resist Formative Dimension

Cover lay & Solder Resist Formative Dimension

8. Gap from stiffener edge to Hole

Gap from stiffener edge to Hole

9. Gold fingers design

Gold fingers design

10. Pattern Cover lay Open Area Specification

Pattern Cover lay Open Area Specification

11. Design in flex pcb bending areas

The bend radius calculation rule is explained in IPC-2223B:

With as main objective to keep constraints lower than the FPCB copper elongation limit.

Design in flex pcb bending areas

12. Flexible PCB dynamic bending

Depending of the radius fold and cycles number needed the FPCB could be adapted. 
For example Hemeixinpcb builds and guarantee FPCB for 100000k cycles into a HDD, and 100k cycles into a mobile phone.

Some data are given for a FPCB double-sided tested with 5mm radius fold: 

  • PI of 12.5μm, copper of 35μm, coverlay of 12.5μm => 20k cycles 
  • PI of 25μm, copper of 17.5μm, coverlay of 25μm => 10k cycles
  • PI of 12.5μm, copper of 17.5μm, coverlay of 12.5μm => 90k cycles

The most Polyimide thickness used for the base material and for coverlay is 25 μm, but for applications needing more cycle in dynamic bending the use of 12.5 μm must be studied with manufacturer. It could rise cycle from 10k to 90k (with copper 17.5μm).

For boards subjected to dynamic bending thinner copper thickness improve the number of cycles. 17.5μm thickness copper is advised and must be studied with manufacturer. Decrease from 35μm to 17.5μm could rise cycle from 20k to 90k (with polyimide 12.5μm) 

In this case bend radius calculation (next chapter) must be done with EB=0.3 %

For boards subjected to dynamic bending, tracks on only one side improve number of cycle. If more layer of copper track is needed, staggered tracks are mandatory.

13. Flexible PCB Static bending

For a natural static bending IPC advise not to place components in bending area but manufacturers tested favorably little and not fragile components. They advise to not place them into bending radius lower than 100mm. The placement in concave (inner radius) bending is less restrictive.
flexible pcb dynamic bending

14. Flexible PCB constraints areas

Each area in a flexible circuit has its constraints; the mechanical designer shall give description details depending on the final requirements.

constraints area
Mechanial constraints areas drawing

While using this guide, keep in mind that the design information provided is only a suggestion. Hemeixinpcb takes pride in manufacturing flex circuits that are considered difficult to build. In most cases, we do build above and beyond the “standard” circuit specifications, provided that the circuit design and type allow for it.

Hemeixin welcomes the opportunity to help you design and manufacture a product that meets or exceeds your expectations. That is why we have established a variety of communication channels to encourage meaningful exchange and dialog.  Please send email to [email protected] If you need some supports.