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.
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.
|A (Min. Width of Marking)||Min. 0.15 mm|
|B (Min. Distance from Land)||Min. 0.2 mm|
|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|
|Mechanical CNC||Laser N.C|
With as main objective to keep constraints lower than the FPCB copper elongation limit.
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.
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.
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.