PCB News - PCB Factory: COB_Process Introduction_Notes_Next

PCB Factory: COB_Process Introduction_Notes_Next

If the wafer has grounding or heat dissipation requirements, [silver glue] is generally used, if not, [anaerobic glue] is used. [Anaerobic Glue] As the name implies, it prevents it from being cured naturally when it comes in contact with air, and does not require high-temperature baking. The use of silver glue requires high-temperature baking to cure. There are two general baking temperatures and times:

Bake at 120°C for 2 hours

Bake at 150°C for 1 hour

Pay attention to the following when choosing silver glue and anaerobic glue:

Anaerobic glue cannot conduct electricity and heat, so you should pay attention to its life when using it.

The reliability of anaerobic glue needs to be further reviewed, and attention should be paid to the possibility of re-melting.

The general COB factories are mostly Low Cost, so most of them use manual mode to produce COB. Another reason is that a small amount of diverse production is not suitable for automation. Of course, if the cost permits, automation equipment can still better control its production quality.

There are two ways to take the die "manually" and stick it on the die pad of the PCB:

Use a small vacuum pen: This kind of vacuum pen is more suitable for large-size wafers, because there is a circular rubber pad on the front end, so too small wafers will be completely covered by the rubber pad, which will affect the accuracy of wafer placement . It should also be noted that the metal vacuum tube cannot directly touch the surface of the wafer to avoid scratching the wafer.

Use silica gel: suitable for small size wafers. Generally, the front end of the toothpick is coated with silica gel, which can be used to stick small-sized wafers, and put it on the solder pad coated with silver glue. The silver glue will stick the wafer to achieve the purpose of sticking.

The silver glue applied on the bonding pads should ensure that 70-100% of the wafer area is adhered to ensure that the wafer will not move in the subsequent process. It should be noted that the silver glue should not overflow the area of the wafer to avoid contamination of the solder joints.
Generally, the maximum wafer bonding rotation angle allowed by the automatic wire bonding machine (Wire Bonding Machine) is 8~10°, while the manual wire bonding machine can allow the maximum angle of 30°. It is best to contact the wire bonding machine (Wire Bonding Machine) circuit board manufacturer to know the maximum capacity of the wire bonding machine.

Wafer storage: Generally, the wafers from the wafer manufacturer will use vacuum moisture-proof packaging; if the wafer has been unpacked, please pay attention to the dust and dirt that should not be exposed, and the surface of the wafer should not be touched by metal objects. . The unpacked wafers can be re-vacuum packed or stored in a nitrogen cabinet to avoid oxidation and any contamination.

To distinguish by the shape of the solder joints, the wire bonding process can be divided into "Ball Bond" and "Wedge Bond". COB usually uses aluminum wire (Al wire) so it is Wedge Bond. According to experience and data, the strength of ball type welding is better than that of wedge type welding, and it can be more expensive.

The advantages and disadvantages of "Ball Bond" and "Wedge Bond":

General COB requires a manual wire bonding machine to repair the bonding wire, because the automatic machine is too expensive, if it is stopped for repairs, the output will be affected.

The general COB does not recommend PCB as a plywood, because the wire bonding machine has a maximum size limit, and the moving range of the wire bonding head is only limited to 4"x4". If you want to print more than two COBs at the same time, We must pay special attention.

As far as I know, the COB process capability can reach a solder joint distance of 90um, but the general COB is more acceptable for a solder joint distance of 100~140um.

There are three methods to test the quality of the bonding wire. The COB process generally only measures "wire pull".

Push crystal

Push the ball

Wire Tension

1. Most COB manufacturers use manual dispensing, because COB belongs to Low Cost, but manual dispensing has the possibility of damaging the welding line and the disadvantages of uneven dispensing shape.

2. The viscosity of epoxy resin is very important.

3. The use of automatic dispensers helps to control the cured shape of the COB epoxy resin.

4. Some epoxy resins need to use a preheating needle tube, because the epoxy resin will reduce its viscosity for a period of time after heating, which will help the epoxy resin flow and reduce the possibility of wire pulling. The recommended preheating temperature of oxygen resin is 60+/-5°C, and the preheating temperature of PCB is 80°C.

5. If the wafer solder joint pitch is relatively small (Fine Pitch), it is recommended to use a relatively low viscosity epoxy resin and use a dam (Dam) around the periphery to prevent the epoxy from flowing everywhere.

6. COB coating is attributed to its quality (air foam, wire failure rate). The liquid epoxy resin of the application form is more willing than the two liquid epoxy resins. But the price of one liquid seems to be more expensive than two liquids.

7. The curing time should be considered. Almost the subcontracting process is applied to cure at 120 degrees Celsius for 2 hours and at 150 degrees Celsius for 1 hour. KHH current curing time is (80 degree Celsius+1hrs)+(110 degree Celsius+2hrs)

Ratio (dry or not)

Stir with a machine

Choose glue

Cost

You can choose Epoxy that has been mixed with OK, but it must be refrigerated at low temperature.

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