Chip on Board Wire Bonding
Bare die wire bonded directly to your printed circuit board, and the right PCB plating for aluminum or gold wire bonding.
Chip on Board Wire Bonding
Chip on board, or COB, mounts a bare semiconductor die directly onto a printed circuit board instead of using a packaged part. The die is attached to the board, then wire bonded to pads on the PCB.
Selecting the right PCB plating for chip on board assembly is crucial. One of the most frequent questions we are asked at Pin 1 Semiconductor is how a printed circuit board should be plated to ensure wire bondability. PCB plating for chip on board assembly must be planned in advance to ensure manufacturability of the device. This planning begins with design rules and selecting the wire bond method to be used. Common wire bond materials include aluminum or gold wire.
- Bare die bonded directly to the PCB
- Aluminum or gold wire bonding
- ENIG, soft gold or ENEPIG plating
- Bondability check on your boards
- Wire pull and ball shear testing
- Die attach in-house
PCB Plating for Chip on Board
The right plating depends on the wire bond method. These are the three common options and how each one bonds.
Aluminum Wedge Wire Bonding and ENIG Plating
One of the most common wire bond methods for COB assembly is aluminum wire bonding. For aluminum wire bonding to a PCB, the plating can be a low cost method known as ENIG, or electroless nickel with immersion gold. With ENIG plating, a PCB with copper traces is plated with a nickel layer, and then the nickel layer has a flash gold layer on top. The flash gold is a barrier layer to protect the nickel from oxidation. The wire bonding connection is actually from the aluminum wire to the nickel plating.
The main advantage of this method is inexpensive plating and relatively easy wire bonding. The disadvantage is that aluminum wire bonding is less flexible than gold bonding and generally requires a larger bond pad pitch compared to gold bonding.
Gold Ball Wire Bonding and Soft Gold Plating
For gold ball bonding to printed circuit boards, a thick layer of soft gold has traditionally been required. In this case the copper traces on the PCB are plated with a nickel layer and then a soft gold layer measuring between 30 and 50 micro-inches. The gold is electroplated and, when plated correctly, provides an excellent surface for wire bonding.
This method has the advantage of allowing fine pitch gold ball bonding. The disadvantages are high cost, and the thick gold layer can cause reliability problems with surface mounted components due to gold dissolution into the solder.
Gold Ball Wire Bonding with ENEPIG Plating
In recent years, electroless nickel, electroless palladium, immersion gold plating has proven to be the best compromise between cost and manufacturability. This method of plating is known as ENEPIG and is commonly referred to as the universal plating method, because it offers good wire bondability and good solderability. It is suitable for RoHS solders as well as eutectic SnPb solder.
The typical plating thickness is 100 to 150 micro-inches of nickel, followed by 4 to 10 micro-inches of palladium, followed by 1 to 2 micro-inches of gold. ENEPIG is suitable for both gold and aluminum wire bonding; however, for gold bonding the plating process must be well controlled.
PCB Plating Comparison
A quick side-by-side of the three common plating options for chip on board.
| Plating | Typical stack | Wire bonding | Relative cost | Notes |
|---|---|---|---|---|
| ENIG | Nickel with flash immersion gold | Aluminum wedge | Lowest | Aluminum bonds to the nickel; generally needs a larger bond pad pitch |
| Soft gold | Nickel with 30 to 50 µin electroplated soft gold | Gold ball, fine pitch | Highest | Thick gold can cause solder reliability problems for surface mount parts |
| ENEPIG | 100 to 150 µin nickel, 4 to 10 µin palladium, 1 to 2 µin gold | Gold or aluminum | Between ENIG and soft gold | Wire bondable and solderable; gold bonding needs a well-controlled plating process |
How We Check Your Boards
At Pin 1 Semiconductor we receive PCBs from many vendors, and wire bondability varies greatly. One simple test is to attach a ball bump on the PCB trace and then do a shear test.
Ball Comes Off Cleanly
If the ball comes off cleanly, there is an issue with the plating.
Palladium Layer Exposed
If the ball comes off and exposes the palladium layer, there is an issue.
Residual Gold Left Behind
If the ball leaves some residual gold wire behind, we are confident the material will work with automatic ball bonding.
We Recommend ENEPIG
For PCB plating with chip on board, Pin 1 Semiconductor recommends ENEPIG as the preferred plating method. It is suitable for both gold and aluminum wire bonding, and offers good solderability for the rest of your board.
Chip on Board Specifications
Have a requirement that falls outside the table? Contact us — chances are we can accommodate it.
| Parameter | Specification |
|---|---|
| Substrate | Printed circuit boards |
| Wire | Aluminum or gold |
| Board plating | ENIG (aluminum wedge bonding) |
| Board plating | Soft gold, 30 to 50 µin (gold ball bonding) |
| Board plating | ENEPIG (gold or aluminum; recommended) |
| Bondability check | Ball bump shear test on your PCB traces |
| Die attach | In-house die bonding |
| Quality control | Wire pull and ball shear testing; digital third optical inspection (3OI) |
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Learn moreReady to move your project forward?
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