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Things to know about flexible LED strip substrates

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Things to know about flexible LED strip substrates


When you look at and compare flexible LED strip types, you probably focus on the color temperature, LED count and pairing the correct power supply for your purchase. But have you considered what the LEDs are mounted on, and how they are all connected? Today, we'll take an in-depth look at the LED strip substrate, and how some of the overlooked specifications and material quality can affect LED strip performance.

What role does the LED strip substrate play?



The LED strip substrate is the circuit-board on top of which the LED chips are mounted. In addition to providing the physical, structural base of the LED strip, the substrate also provides electricity supply through its circuitry, as well as a vital path for heat dissipation.

Structure and materials of a flexible LED strip substrate



The most popular form of LED strip is the flexible substrate type that is sold in 16-ft reels. The substrate type used is commonly known as flexible printed circuit technology (FPC). Flexible electronics have been around for some time now, where they are extremely useful in electronics that have tight, or curved surfaces.

Flexible LED strips capitalize on this existing technology and utilize the same underlying substrate features.  Most often, they will use polyimide (also known as PI) as the material of choice.

Polyimides provide excellent durability and heat resistance despite its flexibility. Thus the polyimide material is critical in providing both flexibility and structural integrity for LED strips.

Starting with a layer of copper which acts as the base circuit, one core layer and two outer layers of polyimide polymer such as Kapton are applied on both sides using a special flexible adhesive. This outer polyimide layers are commonly known as the "coverlay," and can be of a variety of colors. Typically, white is chosen to maximize reflectivity.

The three layers of polyimide provides the copper layer with protection and structural integrity. There are small areas where the copper must remain exposed, however, so that the LEDs and other components can make electrical contact.

Finally, a layer of double sided tape is applied to the back of the LED strip. Most commonly, 3M 200MP is the double sided adhesive used for this purpose.

LED strip with transparent coverlay and 3M adhesive visible:

LED strip with white coverlay:

Copper weight does matter



One of the critical aspects of any electronic circuit is the selection of copper. Quality and purity of copper used for electronic circuits are for the most part standardized, but the thickness of this can vary significantly. Although a measure of thickness, ounces (oz) is typically used to describe the thickness of the copper layer (the technical definition is based on the amount of copper, in ounces, it would take to achieve a certain thickness over 1 sq ft).

When choosing an LED strip light, the copper thickness is something you may want to look out for. Especially for higher power LED strips, we recommend at least 2.0 ounces, and ideally 3.0 ounces and above. All else equal, thick copper is better for the following reasons:

1) Thicker copper means more electricity can flow through the LED strip's circuitry. Insufficient copper can lead to higher electrical resistance and heat buildup, which will ultimately lead to voltage drop and even early LED failure.

2) Thicker copper means faster heat dissipation. The quicker the heat generated by the LEDs can be transferred to the ambient environment, the better the LEDs will perform and last. Copper is an excellent conductor of heat, and therefore having a thicker layer will help significantly in facilitating the transfer of heat away from the LEDs.

Flexible LED strips have poor heat dissipation



The significant downside to flexible LED strip substrates is that their thermal performance is relatively poor. If we look at thermal conductivity numbers, Kapton (polyimide) is 0.12 W/m-K, and the 3M adhesive material is 0.18 W/m-K.

By comparison, aluminum and copper have thermal conductivity values of 205 and 385 W/m-K, and the dielectric layer in metal-core PCBs can reach 2.0 W/m-K. If designed optimally with thermal vias, a two-layer FR-4 PCB's thermal conductivity can essentially be ignored as the heat can be transferred towards the backside copper directly.

There is not much that can be done, but most LED strip products will be designed so that they do not overheat. The downside is simply that the LED strips may be limited in their ability to be run harder, simply because the heat cannot be transferred away quick enough. You can think of this as a Ferrari engine that cannot be run at its full potential because of a radiator with limited effectiveness.

The best way to overcome heat issues is to consider switching to a FR-4 or MCPCB design. This will unfortunately require you to give up the flexibility feature, but will improve your thermal performance significantly.

Also, check your copper trace thickness as you may be burning a lot of electricity as thermal rise in the form of circuitry resistance. If you can't improve the copper specification, you may need to cut down on the number of LEDs you have hooked up on a single LED strip.

Alternatively, you may simply be looking for more brightness per foot - in this case, if you have sufficient space in your extrusion or cove, simply doubling up the number of LED strips (e.g. run 2x rows in parallel) can oftentimes achieve the 2x brightness effect that you are looking for.