The trend for 1W LED is going to chip scale packaging, and like those major available production in the market, the size of the LED Package is in the range of 3 to 4mm. The material is ceramic substrates, and most of them ate epoxy or Flux Eutectic, process.
There are a lot of challenge and see who can manage this entire factor for a successful launch of such a product in the competitive market. There is no perfect process, however to be a pedigree of LED in the market, firstly we should have a stable supply of LED with high efficacy; as if you are a chip design would be a pre-requisite.
Cost and performance are neighborhood but there is always dilemma when cost is a prioritized factor, as we know the ceramic substrate is quite an expensive item for 4” panel is over few tenth of dollars, all depends on how may LED you can put onto this panel, the larger the panel may be having bigger advantage. The cost as a 6” Panel may be additional 15% more expansive than a 4” panel. So if one can manage to use larger panel would be able to drive down the BOM cost lower by a significant amount.
Easy to talk but hard to achieve, a bigger panel will inevitable bring in certain problem such as die bonder constrain as most of the Die bonder is not able to handle 4” wide panel easily.
Warppage of panel is 3X more with 2X larger in size, the average best panel we can get in the market will be having 0.3%-0.5% the width of warppage with size over 4”.
There is a challenge to take care f the warppage.
The package is gong to be thinner and thinner, the thickness of ceramic panel varies from 0.1mm to 1mm. for LED substrates, and the thickness is running between 0.2mm-0.4mm. The thinner the substrates will have higher challenge on the bonding equipment, as the bonding machine should not create any damages or scratches on the thin panel.
And the machine should be able to have up down anvil block or other method to handle the whole panel without damaging it. As we know that the panel is not flat, naturally the die bonding process and glue dispensing process should have an auto height searching function. The force control of the bond head has to be consistent and not to over drive the dies and in certain cases we have to have certain Bond line thickness >5 microns for epoxy process.
High Silver Epoxy is usually have more than 70% of the silver flakes and the carriers will be less in volumes, the epoxy will be viscous, typical visocity will be in the range of 22,000-40,000 CPS. Such kind of thick epoxy will significantly lower down the UPH , this bottle neck for UPH will have to be solved either by increasing the speed of the dispensing process or double the number of epoxy stage. The Later is seems easier as increasing the speed of Dispensing will have adverse effect on the glue size and volume.
For the latest design approach for manufacturing consideration, Silicone liquid molding process is used, in order to have an aligned fudicial mark align as the molding machine alignment method, the Die bonder should have a similar way for Vision inspection method as the moulding machine, That will require a Global alignment of the whole panel, so there will be requiring the die bonder o have a vision system that cab snap images on the fudicial mark at the four edges of the panel.
In order to maximize the number of LED on the panel, the epoxy has to be dispensing exactly the right amount for the dies, too much of the epoxy will bridge the adjacent circuitry and or creating EOD. That is an important process as it is required to be fast, accuracy, and evenly dispensing on the whole panel with high consistency.
Another challenge is bonding accuracy, when more number so dies is place don the panel, the placement accuracy will be shrinked to a narrow range of +/-20 microns or even smaller and die rotation to a range of +/-1 deg.
As overall summary, the die bonding process requires certain equipment which is
-Placement accuracy +/-20 microns
-Die rotation angle +/-1 deg
-Bonding area is 4” wide or more
-Introduce no scratches or damages on the thin crackers
-Able to detect the bonding level at epoxy and bonding stage
-Able to perform global alignment other than the multi-point vision alignment
-UPH 8,000 or more
Such a sophisticated machine is inevitably expensive, glad to know that ASM AD838 series has a cost effective solution on this!