Gold wire bonding is an important packaging process of lighting emitting diode (LED). In this work, we studied the effect of gold wire bonding on the angular uniformity of correlated color temperature (CCT) in white LEDs whose phosphor layers were coated by freely dispersed coating process. Experimental study indicated that different gold wire bonding impacts the geometry of phosphor layer, and it results in different fluctuation trends of angular CCT at different spatial planes in one LED sample. It also results in various fluctuating amplitudes of angular CCT distributions at the same spatial plane for samples with different wire bonding angles. The gold wire bonding process has important impact on angular uniformity of CCT in LED package.
© 2011 OSA
As an energy saving light source, high bright white LED has been widely recognized to be the potential replacement for traditional general lighting source. In recent years, it has more and more applications in our daily life, such as road lighting, headlamps of automobiles, and backlighting for LCD display. So far, most of White LEDs (WLEDs) are based on phosphor-converted principle, in which a blue GaN LED is covered by a yellow phosphor made of YAG:Ce bound in silicone .
The major optical challenges for WLED are in terms of high light output efficiency and angular CCT uniformity. Phosphor coating technology is vital for the abovementioned challenges. To overcome the challenges, corporations such as Cree, Philips Lumileds, and Osram have developed various phosphor-dispersing technologies such as conformal coating, spinning coating and so on [2–5]. Except for the industrial development, a lot of simulation works have been conducted to study the influence of structures and parameters of phosphor layers on optical performances of WLEDs based on Monte Carlo ray tracing. Shi et al. used bi-layered phosphor method to package white LED and found this method caused more than 18% increase in luminous flux compared to conventional random mixed phosphor case at the same CCT , and they also investigated the dependence of luminous efficiency and CCT on phosphor concentration and thickness for WLEDs, and found that LED packaged with lower concentration and higher thickness phosphor had higher luminous efficiency . Our group analyzed the effect of phosphor’s location on LED packaging performance, and suggested an optical structure with plane and remote phosphor location should be a suitable choice for LED packaging with consideration of the luminous efficiency and color uniformity . Sommer et al. also carried out simulations and pointed out phosphor concentration and the geometry size should be precisely adjusted to assure angular-homogeneous white light, and they also identify the impacts of different refractive indices and particle size of phosphor on CCT and luminous efficacies of WLEDs [9,10]. Some other studies on phosphor layer’s geometry optimization are also conducted for the better performance of WLEDs [11,12].
In order to eliminate the inhomogeneity of angular color distribution in WLED with phosphor freely dispersed coating process, our group demonstrated a freeform lens to replace the traditional hemisphere silicone lens, and the angular color uniformity of WLED significantly increased from 0.334 to 0.957 .
Due to its simplicity and low cost, phosphor freely dispersed coating process is widely adopted in WLED production. For any phosphor coating methods as mentioned above, the electrical connection based on gold wire bonding is a must in LED packaging. However, the effect of gold wire bonding process on geometry of phosphor freely dispersed on LED chip, which ultimately affects WLED product’s optical performance, is not considered by most researchers. So far, the LED manufacturers shorten the length of gold wire in order to cut down cost, and also neglect the effect of wire bonding on product performance. So it is necessary to study the impact of gold wire bonding process on angular CCT uniformity of this typical WLED package. In this paper, we will try to study the effect of gold wire bonding on the optical performance of LED by experiments.
2. Experimental processes
In this work, phosphor-converted WLEDs with freely dispersed phosphor coating are employed in study. Samples with different wire bonding angles were packaged. The experimental details are described as follows.
Firstly, one kind of 1 W conventional blue LED chips was used for the packaging. The chip is precisely bounded at the center of LED substrate. Secondly, electronic connections between LED chip and substrate are implemented by wire bonding. Four gold wires are bonded as the electronic connections, as shown in Fig. 1 . Thirdly, phosphor gel, which was the mixture of YAG:Ce phosphor powder and silicone, is doped around LED chip freely to form phosphor layer as color converted element, as shown in Fig. 2 . The mass concentration of phosphor powder was set at 13.04%. Fourthly, to cure phosphor gel, LED samples were placed in a vacuum baking chamber. Fifthly, the geometries of phosphor layers in LED samples were observed and their optical performances were measured. The sample’s angular CCT at point (r,θ,φ) on plane of the top hemisphere is recorded as shown in Fig. 3 . In Fig. 3, the point is specified by (r,θ,φ) in the coordinate system, where the origin is fixed at the center of top surface of copper heat sink; r is the radial distance; inclination angle φ is measured from the fixed zenith direction Z; azimuth angle θ is measured from azimuth axis A; Plane is defined as the plane passes origin and zenith direction Z, and azimuth angle of points on that plane is θ. During the process of CCT measurement, CCTs at points on plane and plane were measured, and r is set as a constant, and inclination angle φ increases from to . Finally, the lens is laid on the substrate, and silicone gel is injected inside and forms the typical LED package, as shown in Fig. 4 .
For the experiments, there are some notifications: a). LED chips with the same characteristics were selected for packaging sample; b). In order to study the impact of gold wires on the optical performance of LED samples, the gold wires were bounded to form different angles and shapes among LED samples.
3. Effect of wire bonding on geometry of phosphor layer
For phosphor gel, surfaces of copper heat sink and gold wire are wettable. After gold wire bonding, the phosphor gel is freely doped on the LED chip on the round substrate. On the one hand, it spreads on the chip surface and top surface of copper heat sink in LED substrate; on the other hand, phosphor gel spreads over the wire surface. During the spreading process, phosphor gel will cover the wettable surfaces until reaching equilibrium state.
In the previous optical simulation of WLEDs, the gold wires are usually neglected and the phosphor layer is considered as a part of spherical cap. However, its real geometry, even without the impact of gold wires, is not exact truncated sphere, and it is closer to oblate spheroid, as shown in Fig. 5 . For the LED samples, the phosphor gel morphology is mainly controlled by the surface tensions. A theory based on minimum free energy is applied to explain microfluidic morphology. Based on Laplace-Young equation, the precise model of phosphor layer geometry is built by our group .
Without the impact of gold wire, it is axially symmetric shape and its surface can be described as follows,Figure 6(a) shows the geometry shape of phosphor gel when not considering the gold wire based on the Eq. (1). It can be seen that the shape is not spherical cap.
In fact, the gold wires have important effect on the geometry of phosphor layer and it cannot be neglected. Due to wetting effect, the phosphor gel spreads along the gold wires’ surfaces. In this case, the phosphor spreading process is complicated, but it can be simulated based on wetting theory. The shape of phosphor layer cannot be axially symmetric.
Based on the flow simulation, we also obtained the phosphor layer shapes at the cases with the gold wire, as shown in Fig. 6(b). From Fig. 6(b), we can see that the geometry of phosphor layer with considering gold wires is not axially symmetric. The effect of gold wire on the phosphor gel shape is existed.
3.2 Experimental observation and comparison
The left and front views of gold wires and phosphor layers in five test samples are shown in Fig. 7 . It can be seen that experimental observation proves the calculation results shown in Fig. 6. The shape of phosphor is not axially symmetric. Thus, it is expected that this will result in the nonuniformity of optical performance in the package, especially CCT deviation.
Figure 8 schematically shows the gold wire shapes and heights in the five samples from the left view. According to the experimental results shown in Fig. 7, the profiles of phosphor layers influenced by the gold wires are also depicted in Fig. 8. It indicates that various gold wire shapes and heights result in different phosphor layer geometries. Compared with the phosphor layer without considering gold wires, it is clear that the shapes of phosphor layers in sample 2, sample 3 and sample 5 are influenced by gold wires seriously, especially at where the gold wires locate. In sample 3, the gold wires are lower than that in sample 1, and the external part is closer to the surface of phosphor, more phosphor gel spreads along gold wires surfaces until surface energy among air, phosphor gel and surface of gold wire becomes balanced. Obviously, this will reduce the height of phosphor layer and change its curvature. For sample 1 and sample 4, the gold wires’ effect on phosphor layer geometry is tiny, and the left and front views of phosphor layers are nearly the same.
To prove the effect of different geometries on the optical performance of WLED, it is necessary to conduct some optical tests on these samples.
4. Optical test results and discussions
Usually, one constant value of CCT is given by integrating sphere optical measurement system for one LED package. Here, CCTs of sample 1, sample 2 and sample 3 and CCTs of sample 4 and sample 5 belong to different ranges and , respectively. Therefore we put samples 1 to 3 into Fig. 9(a) and samples 4 to 5 into Fig. 9(b). The constant value of CCT is not enough to evaluate the package’s optical performance accurately, because the spatial angular CCT in one LED package is not a constant value.
Figure 9 shows the measurement results of spatial angular CCT distributions of different samples, detected by CCT recorder. As shown in Fig. 9, for all the samples, the angular CCT fluctuation at plane is smaller than that at plane in the package. This is due to that the geometries of phosphor layers at plane in all the samples are affected by gold wires more seriously than that at plane , as shown in Fig. 7. Taking sample 5 for instance, the angular CCT decreases dramatically from 7512 K (at ) to 5050 K (at ) at plane , while it decreases from 5719 K (at ) to 5125 K (at ) at plane .
It also can be found that for sample 1 and sample 4, the angular CCT distributions, are similar at plane and plane , whereas, for sample 2, sample 3 and sample 5, angular CCT deviates from each other sharply when inclination angle φ changes from to at the two planes. This is because the phosphor gel shapes at the two planes vary less for sample 1 and sample 4, as we mentioned in Fig. 7.
At the same spatial plane , fluctuating amplitudes of CCT distributions are also different for samples with different wire bonding shapes. Figure 10 shows the angular CCT distributions of sample 1 and sample 3 at the plane . When inclination angle φ increases from to , angular CCT of sample 3 changes from 9904 K to 5976 K, while it changes from 5801 K to 5457 K for sample 1. The fluctuating amplitude of sample 3 is much larger than that of sample 1. This can be explained through the left views of these two samples in Fig. 7. The phosphor gel soaks the surfaces of gold wires apparently and its geometry is influenced more seriously in sample 3, compared with sample 1.
All these experimental results about angular CCT distributions correspond well with the geometries of phosphor layers. The CCT distributions of LED modules, encapsulated with silicone gel and lens, also indicate the similar trend. It illustrates that the gold wire bonding process has an important impact on angular CCT distribution of LED package.
In summary, the effect of gold wire bonding process on the angular uniformity of CCT in WLEDs was analyzed. The experimental study indicated that the gold wire bonding impacts the geometry of phosphor layer distinctly. This induces the different fluctuation trends of angular CCT at different spatial planes in the LED package and results in various fluctuating amplitudes at the same spatial plane for samples with different wire bonding shapes and heights. Thus, during the manufacturing process of LED products, the gold wire shape should be well designed and optimized in order to improve the optical performance of products.
This work was supported in part by National Natural Science Foundation of China (NSFC) Project under grant number 50876038, in part by Ph.D. Programs Foundation of Ministry of Education of China under grant number 20100142110046 and in part by New Century Excellent Talents Project of Chinese Education Ministry under contract number NCET-09-0387.
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