Design Guidance

2022-11-08 BEST

Preparing the Board Outline

Board Outline:

Test your ideas by cutting out a piece of paper in the shape of your proposed board. Use cardboard to represent stiffeners (reinforcements) and rigid areas.
• Begin your layout—but do not route yet!
• Draw the board outline.
• Draw the locations of different thicknesses.
• Do preliminary component placement.
• Determine whether those components need stiffeners (reinforcements).
• Draw the stiffeners (reinforcements) and rigid areas.

Sample outline

The appearance of the circuits should be done without wasting any raw materials and not designing a complicated appearance.

1.The outline designing in a folding way can reduce the raw material wasting.

2.The outline needs to add the copper at the corner to increase the tear resistance.

3.The minimum radius of the inner angle of the contour parts is 1.6mm. If you need smaller one such as 0.8mm, please contact Best Technology. The greater the radius, the more reliable the part is. And the tear resistance will be higher.

4. FPC with dynamic bending requirements, the inner angle bending radius should be more than 1mm.

5. The ends of the slitting and slotting should be machined a hole that diameter is 1.5mm or over 1.5mm.

Please design Fiducial marks at each piece to eliminate the misalignment of SMT caused by FPC size changing after whole manufacturing process if you want to do SMT in panels.

Flex Circuit Thickness:

Avoid unnecessary circuit thickness, which hinders flexible capabilities. The thickness of the flex area is determined by the bend radius needed. If part of the flex circuit needs to be thicker, add a stiffener as reinforcement. The following factors determine the required thickness of a circuit.

• Material thickness
• Design/layout of the materials
• Copper layer count
• Base copper weight
• Adhesive thickness
• Dielectric thickness

There are most popular raw material thickness available in BEST TECH.

1. Currently, the general materials as below:
• Coverlay: PI 1/2mil and 1 mil
• Adhesive: 15um, 20um, 25um and 35um
• Base material: copper: 1/3 OZ, 1/2 OZ and 1 OZ, 2OZ also available
Adhesive: 13um and 20-25um
PI: 1/2mil, 1 mil, 2mil

2. The relationship between adhesive and copper
In general, for 1/2oz copper, 1/2mil coverlay will be needed, so that there’ll be 15um of adhesive between copper and Polyimide, and then the bonding strength will be strong enough. And for 1oz copper, 1mil coverlay will be needed and you can see 25um of adhesive there.

3. Material selection for single layerdual access flex circuit
Please choose 1oz copper for the single layer, dual access flex circuit, because 1/2oz copper is too thin to cause the circuit to break in the fabrication process.

4. For avoiding short circuits, it is best to choose 1/2oz copper for double-side or single-side PCB when the line width and the line space are less than 0.2.

5.For trace width & space is less than 3mil (75um), 1/3 oz or 1/2 oz copper will be recommend.

6. There are four basic stiffeners available in BEST TECH.
a)PI stiffeners: thickness 1mil - 7mil
b) PET stiffeners: 2-6mil
c) FR-4 stiffeners: 12mil-200mil
d) Stainless steel stiffener: 4-8mil

Here are Stiffener thickness available in Best Technology.

Stiffener (Reinforcement) Thickness Availiability
PI 0.05mm, 0.075mm, 0.1mm, 0.125mm, 0.15mm, 0.175mm, 0.2mm, 0.225mm, 0.25mm
FR4 0.1mm, 0.2mm, 0.3mm.........2.0mm (from 0.1mm to 2.0mm are available)
Steel 0.1mm, 0.15mm, 0.2mm, 0.25mm, 0.3mm
PET 0.188mm(without adhesive), 0.235mm(with adhesive)

PI, FR-4 and stainless steel stiffeners are generally used in high-temperature resistant FPC. PET is used in low-temperature resistant FPC.

7. PSA (Press Sensitive Adhesive)
The most popular ones used in Best Technology are 3M, Tesa, and there’re normal temperatures such as 3M467, 3M468, Tesa 4972, and high temperatures such as 3M 9077, 3M 9079, Tesa 8853, and high-temperature one can undergo lead-free reflow oven.