Some time ago, I read two articles about the COB in the expert saying that the APP has already been forgotten, but it is still quite good to say two words when not busy. Of course, it is not a question of right or wrong for the two authors to argue, just to reiterate some basic common sense. My Chinese is not too good, please forgive me. This article communicates with you from the advantages and disadvantages of COB.
1. Advantages of COB: good heat dissipation and low cost
As we all know, there are three ways to dissipate: 1. conduction; 2. convection; 3. radiation. But for LED packaging, the first consideration is the first (ie conduction). The relevant laws of physics are:
Heat transferred = thermal conductivity x heat dissipation area x temperature difference / heat conduction path length
It can be seen from the formula that the difference in thermal conductivity, heat dissipation area and temperature is better, and the smaller the heat conduction path length, the better, that is, the more heat transferred, the better the heat dissipation effect. The greater the thermal conductivity, the smaller the thermal resistance. For multi-layer thermal material overlap, the above formula should be applied multiple times.
When comparing two structures with the same function (for example, comparing SMD and COB), they can be considered on a case-by-case basis. It is important to note that when comparing a parameter, other factors must be the same. When comparing the heat dissipation capabilities, compare the same heat sources, such as 10W SMD and 10W COB. The heat sink must have the same material and area. This is the "Apple vs. Apple" to distinguish between good and bad. If it is different, it is "comparison between apple and orange" and there is no comparability. ? The comparison between COB and non-COB is given below.
(1) COB: As we all know, the heat of LED is mainly generated by P-epitaxial layer.
Figures 1 and 2 show the two basic structures of COB: Figure 1 is a flip-chip COB, and Figure 2 is a formal COB. I am not quite sure what kind of domestic is more. Flip COB is more difficult to do, and the heat dissipation effect is better. There are multiple chips on one heat sink, and the chip is covered with phosphor.
tell a story. I was at the first meeting in 2004 and someone reported COB. At that time, the COB etched a groove on the silicon wafer and placed the chip in the groove. However, the process is more difficult and there is no promotion. According to this concept, we designed a QFN structure similar to the semiconductor industry and a COB structure without a groove. In 2006, I first saw the COB product, which was just launched, with the main purpose of reducing costs (including improving production efficiency) and solving the heat dissipation problem of high-power devices (those that just started producing 1W chips). Compared with the K-2 at that time, COB has obvious low cost and good heat dissipation. Compared with the SMD at that time, the COB has little advantage in heat dissipation, but the cost advantage is still very large.
(2) Non-COB with package structure: more package base (extra thickness), poor heat dissipation, high cost
As mentioned above, the controversy points of the previous two authors can be considered from the motivation to develop new products (such as developing COB). As an engineer for nearly 20 years, I think there are basically two motivations for developing COB:
(1) Reducing the material cost of the package at that time, increasing production efficiency and reducing production costs;
(2) Improve heat dissipation performance. An added bonus is the partial elimination of ghosting.
The debate now is whether the new product has achieved its original purpose? Obviously it is achieved.
The question is coming, then why do some people find that it is not good, but the company continues to produce, and the market continues to accept? There are two reasons for this:
(1) From a market perspective, there may be no new products to replace it in the short term, or the price of new products is too high;
(2) From the perspective of the manufacturer, or the cost of the new product is too high, or the process is too difficult, or the investment is too large to be put into production, and has to give up. This is why international companies are now striving to develop better quality, more efficient COBs in order to maintain their advantage and market share for Chinese companies.
Twenty years ago, when I first debuted, as an engineer, I just wanted to think about technology. But the boss will consider the cost, whether it is at home or abroad, most of the reason.
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