Tag Archives: Printed Circuit Board Fabrication

BGA-HDI

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

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 PC 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 PC 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.

3d-printed-pcb-prototype

Accelerate your New Product Development with rapid PCB assembly prototyping

The time from concept to prototype has accelerated remarkably in the past decade. 3D printed prototype components in a wide variety of materials are available in hours. Machined or sheet metal components are available from rapid prototype shops in only one or two days.

Prototype Printed Circuit Board Fabrication and assembly companies like PNC have followed this trend towards faster prototypes and can now provide complete assemblies in less time than ever before. PNC can fabricate and deliver a bare 10-12-layer PCB in just three days, and a simple double-sided board in just 24 hours.

However, even with the streamlining of PCB fabrication, the fully assembled PCBA is often the longest lead component in prototype designs primarily because of the sheer number and variety of passive and active components to be purchased and the demands of accurately placing and soldering those components. Sourcing the components on a typical PCBA BOM can take days in the best case and weeks in the worst case. Setting up and running the assembly job can add another few days, especially for double sided PCBs, and PCBs with a combination of surface mounted and through hole components.

Fortunately, there are some things that a product development team can do to reduce PCB assembly lead time.

First, do everything possible to reduce the impact of long component lead times. Plan to order the components as early as possible in the circuit design process. Deciding when to order components requires balancing the costs of scrapping some components as the design matures vs. the benefits of reducing the lead time for an assembled PCBA by days or weeks.

Second, reduce the time required to set up and build the prototypes by working with a full-service company like PNC. PNC has the capability to both fabricate the bare PCB and assemble the components. This means that the PCB fabrication team and assembly team can save time by working in parallel. While the PCBS are being fabricated, PNC’s engineers can create pick and place data, solder paste stencils and program the assembly equipment. When the PCBs are finished and the components arrive, everything is ready to begin assembly immediately.

The third way to save time with PCB prototypes is to minimize the number of PCB prototype iterations. Saving a full printed circuit board assembly prototype cycle is the most effective way to reduce the time from concept to mature design.

One way to reduce design iterations is by testing circuit designs as early in the design process as possible by building “Works Like” prototypes. “Works Like” prototypes are usually combinations of development kits, large one or two layer PCBs with larger SMT components that can be soldered by hand and various types of breadboards. In addition to testing the circuit, a “Works Like” prototype gives software developers an early platform to start developing code and debugging the circuit design. The result of testing early with rough prototypes is that you fix problems before you have invested the time in the full layout and prototyping process.

In parallel, the mechanical engineers can optimize cable routing and connector placement by printing 3D models of the PCBs, then epoxying actual connectors to the board model. This is an effective way to quickly try different options for cable routing using actual cables and connectors, since it is difficult to simulate the way actual cables behave with CAD software.

Experienced electrical engineers know that it is often poor connector access or cable interference that drive Printed Circuit Board layout redesigns as often as issues with actual circuit performance.

Once the circuit has been tested with the “Works-Like” prototype, and the board layout has been tested with 3D printed models, the last way to save time is to work closely with the PCB manufacturer to make sure that the PCB fabrication files are clean and complete, and that the BOM is accurate and matched with the circuit and layout to avoid placement mistakes.

This is another reason to select a full-service prototype pcb manufacturer like PNC. PNC can be a partner during the layout and design process, though fabrication and assembly ensuring the final design can be translated into a working prototype in the least possible time.