12 Flexible Printed Circuit Boards
12.3 Design Considerations in Flexible Circuits
12.3.1 Difference in Design Considerations of Rigid and Flexible Circuits
Most of the design rules for rigid PCBs have to be applied for the design of flexible PCBs. There are, however, a few exceptions plus some new considerations to be taken into account. A few of them are given below.
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Current-carrying Capacity of Conductors: Because of less cooling capability by the flexible board itself (when compared to rigid PCBs), sufficient conductor width has to be provided. A guideline for selecting conductor widths for currents of more than one ampere is given in Figure 12.8. Where several conductors with a high current are placed opposite or neighbouring each other, the heat concentration has to be taken care of by giving additional conductor width or extra spacing.
0 0.062 0.1250.250.370.50 0.75 1.0 1.5 2.0 2.5 3.0 3.54.0 5.0 6.0 7.5 10.0 12.5 15.0 17.5
34 32 30 28 26 24 22
45°C
30°C 20°C 10°C
0 1 5 10 20 30 50 70 100 150 200 250 300 350400450 500 600 700 0
0.001 0.005 0.010 0.015 0.020 0.030 0.050 0.070 0.100 0.150 0.200 0.250 0.300 0.350 0.400
Cross section areain squareMils
Example #1:A current of 1 amp withẵoz.copper and 30°C temperature rise willrequire a conductor width of 0.040".
Example #2:A conductor with width 0.140", etched from 1 oz.copper (0.0014") willproduce a temperature rise of 10°C at 2.7 amps.
ẵ0z/ft (0.0007")2 1 0z/ft (0.0014")2 2 0z/ft (0.0028")2 3 0z/ft (0.0042")2 Example#2
Example#1
Wire gauge equivalent (AWG)
CurrentinamperesConductorwidthininches
Fig. 12.8 Guidelines for selecting conductor size (www.minco.com)
Contours: Wherever possible, rectangular shapes are preferred because of the better base material economy. There should be sufficient free border space near edges due to the possible dimensional
changes with the base materials. Inward looking corners in the contour should be rounded; sharp inward corners could initiate tearing of the board.
Areas of small conductor widths and spaces should be minimized as much as possible. Conductor width should transition from fine lines in tight areas to wider widths where geometry allows.
Conductors terminating at PTH vias or component mounting holes should have a smooth fillet transition from the trace into the pad as shown in Figure 12.9. As a general rule, any transition from a straight line to such features as corners or different line widths must be done as smoothly as possible. Sharp corners constitute a natural place for stress to accumulate and for conductor defects to occur.
Improved
Conductors and spaces too narrow Borders toolarge
Incorrect
Fig. 12.9 A smooth transition from trace to pad reduces stress and improves reliability. In the top view, the conductors and spaces are too narrow, and the borders too large, the bottom view shows an improved design
Bending: As a general rule, the bending radius should be designed as wide as possible. The possibility of undergoing many cycles is also further improved with thinner laminates (e.g. 50 mm foil instead of 125 mm foil) and larger conductor widths. If subjected to a higher number of bending cycles, single-sided flexible PCBs, in general, show a better performance.
Solder Pads: Around the solder pad, there will be a transition from flexible to rigid material. This zone is highly prone to conductor breakage. Solder pads are therefore, avoided in active bending zones.
Good Acceptable Bad
Good Bad
CoverFilm
Fig. 12.10 Shape and masking of solder joints (a) shape of solder pads (b) solder joint masking with cover film (after Bosshart, 1983)
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The general shape of solder pads should be tear-like (Figure 12.10) and the cover foil must mask the solder joint close by.
Hardboard Stiffeners: The combination of flexible PCBs with adhesively laminated hardboard stiffeners has become extremely popular and cost-effective in the bulk production of small electronic equipment like pocket calculators. The flexible PCB is mounted on one piece of hardboard (e.g.
grade G-10) with suitable slots for separating at a later stage. This is illustrated in Figure 12.11.
After component assembly and wave soldering, the cutting operation divides the hardboard into different parts, thereby facilitating folding into the planned shape.
Flexible PCB, folded Components B A
C A
B
C
Flexible PCB
Hardboard Cuttingline for finalhardboard cutting
Dividing slots
(a) (b)
Fig. 12.11 Use of hardboard stiffeners (a) Board as it goes for assembly and wave soldering (b) Board after final hardboard cutting and bending
The above special requirements indicate that designing a flex circuit is only a few steps away from designing a hardboard. However, the important design differences to be kept in mind are:
a Three-dimensionality of a flex circuit is important as creative bending and flexing can save space and layers
a Flex circuits both require and permit looser tolerances than hard-boards a Because since arms can flex, they are designed slightly longer than required.
The following design tips are useful for minimizing circuit cost:
a Always consider how circuits will be nested on a panel.
a Keep circuits small; consider using a set of smaller circuits instead of one large circuit.
a Follow recommended tolerances whenever possible.
a Design unbonded areas only where they are necessary.
a If circuits have only a few layers, stiffeners can be far less expensive than designing a rigid flex circuit.
a Specify 0.0001" of adhesive on the cover material per 1oz of copper (including plated copper).
a Building circuits with exposed pads and no cover layers is sometimes less expensive.
12.3.2 Step-by-step Approach to Designing of a Flex Circuit
The following steps are guidelines to design a high quality, manufacturable flexible printed circuits (Minco Application Aid 24,):
a It is always good to start with a study of the available literature that is applicable to the intended application. The most useful literature is either the IPC or MIL standards, for example, if the circuit is intended for applications in the military/aerospace field, reference may be made to IPC-6013 and IPC-2223 or MIL-P-50884.
a Define the circuit parameters according to the package that uses the circuit. It is always helpful to cut out a paper template to represent the actual circuit. Experiment with bending and forming the template in order to achieve maximum efficiency. Design a circuit for maximum ‘nesting’ in order to have as many circuits as possible on a panel.
a Determine the wiring locations and conductor paths. This will determine the number of conductor layers. The circuit cost generally rises with the layer count. For example, two double-layer circuits could potentially be less expensive than one four-multi-layer circuit.
a Calculate the conductor width and spacing according to the current capacity and voltage.
a Decide what materials to use.
a Choose the method of termination and through-hole sizes. Evaluate the bend areas and methods of termination to determine if stiffeners are needed
a Lay down the methods of testing. Avoid over-specification to reduce cost.
12.3.3 Designing for Flexibility and Reliability
The flexible circuits are classified by the type of flexing they will undergo during assembly and use (Corrigan, 1992). There are two types of designs, which are discussed below.
Static Designs: Static designs are those which are flexed or folded only for assembly or on rare occasions during the life of the product. Single- and double-sided as well as multi-layer circuits can be folded successfully for static designs. Generally, the bend radius of the fold should be a minimum of ten times the total circuit thickness for most double-sided and multi-layer designs. Higher layer count multi-layer circuits (eight layers and more) become very rigid and are very difficult to bend without problems. Therefore, they can be designed to have zones with fewer layers for folding.
Double-sided circuits requiring tight bend radii are designed to have all the copper traces in the fold area on the same side of the base film. By removing the cover film on the opposite side, an approximation of a single-sided circuit will be achieved in the fold area.
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Dynamic Designs: Dynamic circuits are intended to be flexed repeatedly throughout the product lifecycle, such as cables for printers and disc drives. In order to get the highest flex life for a dynamic circuit, the concerned part should be designed as a single-sided circuit with the copper in the neutral axis. The neutral axis is the theoretical plane at the centre of the layers of materials that make up the circuit. By using the same thickness of material on either side of the copper, the base film and cover film, the copper will lie in the exact centre and be exposed to the least amount of stress during bending or flexing.
Designs requiring both high dynamic flex life and high density multiple layer complexity can now be achieved by connecting double-sided or multi-layer circuits to single-sided circuits with anisotropic (Z-axis) adhesive. The flexing takes place only in the area where the assembly is single- sided. Outside of the dynamic flex area, isolated multi-layer zones exist for complex wiring and component needs, without compromising flexibility.
Although flex circuitry is expected to fill applications that require the circuit to bend, flex and conform to fit the specific use, a large percentage of failures in the field are a result of these flexing or bending operations. Using flexible materials in the manufacturing of a printed circuit does not in itself guarantee that the circuit will function reliably when bent or flexed, particularly in dynamic situations.
Many factors contribute to the reliability of a printed flex circuit that is formed or repeatedly flexed.
All these factors must be taken into account during the design process to ensure that the finished circuit will function reliably. Some tips to increase the flexibility for a reliable operation are:
a A circuit with two or more layers should be selectively plated to improve dynamic flexibility.
a It is advisable to keep the number of bends to a minimum.
a Stagger conductors to avoid the I-beam effect and route conductors perpendicular to a bend as shown in Figure 12.12.
a Do not place pads or through-holes in bend areas.
a Do not place potting, discontinuities in the cover, discontinuities in the plating or other stress concentrating features near any bend location.
It should be ensured that there are no twists in the finished assembly. Twisting can cause undue stress along the outer edges of the circuit. Any burr or irregularity from the blanking operation could potentially lead to a tear.
a Factory forming should be preferred.
a Conductor thickness and width should remain constant in the bend areas. There should be variations in plating or other coatings and preferably no conductor neck down.
a It is a common practice to provide a slit in a flex circuit to allow different legs to flex in different directions. Although this is a valuable tool to maximize efficiency, the slit represents
Staggered Conductors
I-beam arrangement
Fig. 12.12 Staggered conductors vs. I-beam effect
a vulnerable point for a tear to start and to propagate. This can be prevented to place a drilled relief hole at the end of the slit as illustrated in Figure 12.13 and to reinforce these areas with hardboard material or a patch of thick flex material or Teflon (Finstad, 2001).
Another possibility is to make the slit as wide as possible and to place a full radius at the end of the slit (Figure 12.14). If reinforcement is not possible, the circuit should not be flexed within one-half inch of the end to the slit.
Fig. 12.13 Provision of a drilled relief hole at the Fig. 12.14 The slit is made as wide as possible and end of a slit placed a full radius at the end of the slit