#1-LED LEAD FRAME
Part - 3
Material Selection 3: Design
LED lamps are directly or indirectly exposed to the air with a certain degree of moisture. Imagine there’s a poorly designed structure that is not moisture resistant enough, and the moisture penetrates into the lamp. Now that it’s been lit up for long hours and is wet, the resin falls off from the metal substrate, pulls apart the welded wires and eventually cuts the connected circuit. A deficient design may restrict the fields, places, and conditions of product usage. That’s why LED designers and manufacturers have to always focus on the moisture-resistance structure. That requires best compatibility of resin and metal substrates, advanced injection molding technology, appropriate molding temperature and molding process time.
Moisture permeates into an LED lamp via two paths, either from the fringe of resin-covered copper, or from the surface. Copper surface is exposed both upper and lower sides, making the latter path way much easier.
So, how do we solve this problem?
Solution 1: Extending the borderline of the metal substrate.
By tamping the metal’s fringe into complex curves, the fringe gets longer, which makes it difficult for the moisture to go in. Certainly, the more complicated process it goes through, the more it costs, and the higher demand there is for the metal substrate’s performance.
Solution 2: Roughening the metal substrate.
Coarsened surface of a metal substrate
Today, the idea of increasing the coarseness of metal surface is commonly accepted and adopted. It helps extend the path for the moisture to go through, thus impeding the flow of the moisture.
Solution 3: Reducing the substrate’s contact area with resin.
Cross section view and vertical view of a substrate with punched holes
Metal substrate with punched holes on it
The idea of narrowing contact area between metal substrate and resin is also commonly applied to current structure designs. This theory is realized through creating holes on the metal surface in different sizes and shapes. More holes bring greater results in improving moisture resistance in two ways. First, the resin above and below the substrate can stick much closer through the holes, making the metal fastened tighter. Second, it helps reduce the contact between the metal and the resin so that there won’t be cracks within.
Solution 4: Packaging side leads with plastics.
We can also cover the side leads with plastic and leave only the bottom lead exposed. In this way the path for penetration is been prolonged and the moisture permeation is prevented efficiently.
Structure Design Impacts Light Output
Package design and material allocation of an LED lamp’s lead frame should be carefully dealt with because they have tremendous impacts on the product’s performance in heat dissipation, reliability and luminous efficiency.
More specifically, the shape and size of the reflector cup from bottom to top, height of the cup, whiteness of the resin, reflectivity of silver coating and placing of the die are all significant in the sense that they influence the light brightness, light intensity, half-value angle and the emission angle. Structure of the frame determines shape and thickness of the resin, whose light transmittance and refractivity are also the dominants in the die’s light extraction efficiency.
Again, heat dissipation, reliability and luminous efficiency. These three factors should be attentively considered when designing the lead frame structure, otherwise the product’s longevity, performance and color rendering ability will be hugely compromised.
#1-LED LEAD FRAME