Installing flash LEDs on flex circuits

For the mobile market, some PCB assemblies have been converted to flex circuit assemblies, in part because flex circuit assemblies can be twisted or bent to application needs.

Flex circuits offer the same advantages as conventional printed circuit boards: quality, reliability and high density.

However, the most important attribute that has encouraged designers to opt for flex circuit technology is the capability of the flex circuit to assume three-dimensional configurations. These circuits can flex during installation, maintenance and use, and can save up to 70% of the space or weight of conventional wiring.

Assembling components on flex involves a cleaning process, soldering process and handling process. Bending close to the edge of the component may cause solder joints to crack and subsequently cause open-circuit failures.

A flex circuit is made from a flexible polymer film laminated to a thin sheet of copper that is etched to produce a circuit pattern. The circuit can be manufactured in several different configurations, including single-sided, double-sided, multilayer and rigid circuits.

In single-sided construction, one conductive layer is present with copper laminated with a polyimide cover film.

In double-sided construction, two conductive layers with an intermediate insulating layer are formed and laminated with a polyimide cover on both sides. Access holes or exposed pads may be on either or both sides.

Dual access is similar to single-sided flex, however it provides access to both sides of the copper layer.

Multi-layer can be three to 10 layers, consisting of multiple layers of single- and/or double-sided material laminated with thermoset adhesive.

Wimpy is a combination of single- and double-sided flex. Certain areas will have double-sided construction and the dynamic flexing area will have single-sided construction to achieve maximum flexibility.

Rigid flex is by far the most difficult construction in the PCB or flex circuit industry. This construction uses a combination of flex circuit materials and rigid printed circuit materials. The flex is laminated with thermoset adhesive onto a single- and/or double-sided FR-4 clad material.

Conventional flex assembly is too soft to support heavy components, thus rigid flex is the recommended assembly when using LEDs such as Avago Technologies flash modules which have substantial mass and need better support.

The solder joint will be very fragile when there is a bend near the joint. An accidental bend of the flex close to the solder joint will cause cracks, and thus open-circuit failure.

To provide the best support for the LEDs, the best recommendation is to use a hybrid combination.

This consists of rigid boards but has the design flexibility of flexible circuits. The boards provide a higher component density and better quality control.

Designs can be rigid where support for components is needed and flex around corners and in areas requiring extra space.

Figure 1 is an example of the recommended rigid flex assembly.

Figure 1: Rigid flex assembly.

Figure 2: Example of copper spreading on flex.

Note: Whenever multiple flex layers are used on folding boards, allowance must be made for the inner flexible layer to kink when the board is folded.

Try to get a balance of copper on both sides of the foil, otherwise it will tend to warp during manufacturing and the weaker side of the flex will suffer more stress when bent.

An appropriate soldering process is needed to form a good solder joint. For example, insufficient heat to the solder paste will cause a problem with dry joints.

There are various soldering techniques, such as manual soldering, reflow soldering, hot bar soldering and many others.

Hand soldering is not recommended, especially for surface mount LEDs, since hand soldering is highly dependent on operator skill level to achieve a good joint. There is the possibility of overheating the LED if there is poor process control and/or operator skills.

The reflow soldering process is strongly recommended.

Figure 3 is the recommended reflow soldering profile for the Avago 1X3 flash LED.

Figure 3: Recommended reflow soldering profile.

The bending process usually takes place after all the components are mounted on the flex circuit (see Figure 4).

Figure 4: Bending process on flex.

Notes:

1. LED edge/solder paste edge is the weakest point on copper flex.
2. Any bending at the edge may cause broken solder joints/traces.
3. During assembly, do not bend this weak point.
4. An epoxy coating under the flash LED between the two solder terminals may help to increase adhesion to the flex.
5. Solder pads or mounting holes should be placed at least 2.54 mm away from the bend area.
6. Very thick copper should be avoided on flex layers to minimise stress during bending.
7. Permanent bend flex tails or tails with a sharp bend near the rigid part should be secured with adhesive.
8. If the circuit is used for a continuous flex application, plating at the bend area is not recommended.
9. The maximum degree of bend in rotary-draw bending is 180° (Figure 5).

Figure 5: Bend radius definitions.

Table 1: Recommended bend radii.

The minimum bend radius is a function of the thickness in the flex area. Table 1 shows the recommended bend radii as recommended by MIL - P - 50884 C.

Before bending on flex, a jig is needed to support the component on the flex to avoid bending near the solder edge.

Note: During clamping, do not pull or adjust the flex. Only bending is allowed.

After bending, slowly pull out the jig when the bending is done.

Avago Technologies
www.avagotech.com