Tag Archives: PC Board

What’s a HDI Printed Circuit Board?

Circuit board fabrication, HDI Printed Circuit Board, PC Board, PC Board fabrication, pcb assembly, , , , , , , ,

HDI stands for High Density Interconnect. HDI PCBs have finer traces and trace spacing, laser drilled micro vias and higher connection pad density. Its two chief advantages are that it permits the use of fine pitch BGAs and it reduces the number of PCB layers required because the finer traces and smaller vias allow more circuitry in a smaller area.

Narrow trace widths mean higher circuit density

At PNC standard PC Board fabrication uses a minimum trace width of 5 mil, with a 5 mil space between traces (5/5mil) PNC’s HDI trace widths can be as narrow as 3 mil with 3 mil spacing.  These finer traces allow 160% more traces in the same real estate. 3/3mil spacing will also allow two traces to escape between pads of a standard BGA, meaning less PCB layers are needed to fan out the pins from the BGA.

Microvias are the enabling technology for HDI

Narrow trace widths used in HDI PC Board are a result of the gradual refinement of photolithography and etching technology.  Microvias on the other hand, are a revolutionary innovation driven by the development of high powered lasers that can be controlled accurately enough to ablate a 3 mil hole through the surface layer of copper and underlying laminate, without damaging the underlying layer of copper.

The minimum hole size for PNC’s laser drilled microvias are 3 mil and the minimum pad diameter for the microvia is 7 mil.  Pads for laser drilled holes can be smaller than for mechanically drilled holes because of the location accuracy of the laser drilled hole.  There is no mechanical deflection of the drill bit to account for.  The laser drilled holes can be fully copper filled and planarized flat, so they can be used as pads for fine pitched BGAs with 0.4mm or smaller spacing. Using microvias as pads allows the signal trace to fan out by going straight down and out to an inner layer of the printed circuit board.

The biggest limitation with microvias is the aspect ratio of the holes.  Where a drilled through hole can have a 10:1 depth to diameter aspect ratio, a laser drill can achieve no more than around a 1:1 aspect ratio.  This means that the smallest microvia can only connect two adjacent copper layers. A larger diameter microvia can penetrate two layers. To connect deeper layers, the designer must stack vias one directly atop another.

Laser drilling of the microvias changes the way PCBs are fabricated and gives the designer flexibility that they do not have with through hole vias.  In a standard drilled PCB, via holes are drilled and plated after the PCB fabrication stack-up is completed.  Because the microvias can only bridge two or three copper layers, the microvias must be drilled and plated at each lamination step.  This means that microvias can be fully buried between layers, stacked or staggered to allow the microvia to connect multiple layers of the stack up.

The major space saving advantage of the microvia technology is that vias can just connect traces that need to be connected, rather than taking up real estate all the way through the PCB the way a through hole via does.

The Printed Circuit Board designers at PNC take advantage of this by locating the power and ground layers at the top of the stack up.  Since all active components access power and ground, sometimes through multiple pins, having the power and ground layers directly below the component layer allows all those connections to be made directly by microvias.  This leaves the component layers and layers beneath the power and ground layers completely unobstructed for signal routing. This has the added advantage of reducing parasitic capacitance because it eliminates the circuit stubs caused by plated through holes.

Two sided boards are typically fabricated with a combination of through holes and microvias.   Though holes can be drilled just through the core, connecting the stacks on the top and bottom of the board from the lowest layer, or through holes can be drilled through the entire stack directly connecting the traces on the top and bottom component layers.

 

HDI PCBs are a necessity when using fine pitched BGAs, but they can also reduce cost on PCBs without fine pitched BGAs because of the reduced layer count.  On your next PCB design, talk to the experts at PNC.  They can help you determine if HDI technology is can reduce your PCB cost by reducing layer count and shrinking the PCB size.

BGA-HDI

Better BGA routing on a Printed Circuit Board with High Density Interconnect

Circuit board fabrication, PC Board, PC Board fabrication, PCB Fabrication, Pga Capabilities, , , , ,

One of the technologies that have allowed electronic products to shrink in size and provide increasingly higher performance is the ball grid array (BGA) IC package. The BGA allows higher density Printed Circuit Board layouts because of simple geometry. The number of pins that can be accommodated on the perimeter of a quad pack increases linearly with package size. The number of connections that can be accommodated on a BGA increases with the square of the package size. BGA packages now routinely exceed 1000 connections, and the ball pitch has shrunk from 1.0 mm to .8 mm to a growing number of devices now available in the micro-BGA format with a ball pitch of .65 mm, .5 mm and smaller.

The decrease in BGA ball pitch would not have been possible without the improvements in PCB fabrication. These improvements, collectively called High Density Interconnects or HDI, give companies like PNC the capability of creating traces as narrow as 0.0762 mm (3 mil) and vias with annual rings as small as .25 mm (10 mil) HDI PCB fabrication has given designers greater flexibility in routing BGA devices down to a pitch of .65 mm The HDI technology is essential for devices with ball pitches less than .65mm

Routing the hundreds of connections from a typical BGA is called BGA breakout, and it can be a major layout challenge. For this reason, many designers place the BGAs into the layout first and fan-out the connections from each pad to a stub trace. This allows the designer to adjust the routing of individual pins under the BGA without rerouting the entire Printed Circuit Board. Another reason the BGA should be placed first is that the BGA breakout will likely dictate the number of layers needed in the PCB stack-up.

The breakouts are typically a repeating pattern, with the traces for each row of balls around the perimeter routed similarly. Most BGA manufacturers will provide sample breakouts, and some high-end tools will automate this breakout process. Most BGAs use similar fanout approaches, the fanout differing only in the package specific routings for power and ground. With standard PC Board fabrication technology there really are not a lot of fanout options. Here is the typical approach used for BGAs with pitches down to .65 mm highlighting some of the advantages of PNC’s HDI fabrication technology

Routing the first perimeter row of the BGA is easy; the traces come straight out from the pads.

The traces for the second row pass between the pads of the first row. If the ball pitch is greater than .8 mm an HDI PCB fabricator with the capability of creating 3mm pitch traces can fit two 3mm traces with 3mm spacing between the pads in the outer row. This allows the first three perimeter rows of pads to be routed on the top layer.

Subsequent rows are routed using a feature called a dogbone. The dogbone has a pad at one end and a via at the other, separated by short trace. This prevents the via from wicking solder from the ball pad, starving the solder joint. It is also recommended to cover or “tent” the dogbone via with solder mask. The dogbone is typically oriented at 45 ° so that the via can be located in the center of each four pad grid. The via takes the signal trace to the next level where it is routed out between the other vias, similar to what was done on the top layer.

the following number of board layers typical are needed for each perimeter row of pads

board layers
board layers

This table demonstrates that using an HDI Circuit board fabrication process, even for a 1.0 mm or .8mm pitch BGA can result in the need for fewer signal layers, because two traces can be passed between each pad. The HDI fabrication process also allows the dogbones to be placed in line with the grid instead of diagonally, which allows two traces to pass between vias on the 2nd and 3rd layers

For smaller pitch devices PNC’s HDI fabrication techniques become essential. For ball pitch spacing of .65 mm and .5 mm the only way to create a fanout is using the 3 mil traces and 10.68 mill dia. vias allowed by HDI. The 3 mil trace and 3 mil trace spacing allows a single trace to just fit between .5 mm pitch pads.

The latest micro BGAs used in devices like phones and smartwatches have pitch spacing below .4 mm. The pitch spacing is so close that traces no longer fit between the pads. BGA breakout requires via in pad techniques, with the filled microvias routing the signals straight down and then out. Depending on the number of perimeter rows, blind and buried vias may also be needed.

If you are using a BGA in your design, using HDI design rules for fabrication can simplify the breakout and reduce the number of PCB layers needed. PNC engineers can help you understand what is possible with HDI Printed Circuit board fabrication.
The last thing to know about designing with BGAs is that process yield, and reliability are very process dependent. When selecting a
Pga Capabilities it pays to select PNC. PNC has the equipment and expertise to manufacture your most challenging BGA designs.