IC Packaging and SiP Blogs

Most package substrates are designed as they will be placed onto the host PCB if the package were mounted on the top side. This means that the BGA’s balls are on the bottom layer of the cross-section. Your dies are mounted on the top.

For wire bond dies, these are still placed chip-up (die pads on facing away from the substrate). But, for flip-chips, the chip is placed chip-down. It is flipped over and mounted with the bumps on the top surface of the top routing layer of the host BGA. When the package is complete, a library symbol is created from the BGA for use by the PCB designer.

What happens if you want to design your package from the opposite perspective? With the flip-chip mounted chip-up on the bottom layer of the cross-section and the BGA balls facing up on the top layer, the package design is viewed in an orientation common to the IC; this can allow for easier communication with the IC design team, as you’ll both see the bump pads in the same relative layout. Allegro Package Designer makes it easy to design in this manner.

There are two primary approaches to achieve this. Care must be taken in both cases to ensure that both your design process and your eventual library parts are correct. The last thing one wants is to get to the very end, ready for manufacturing, and find that your PCB library part doesn’t match up with your package substrate!

How, then, can you take on this task?

BGA Components on Top

To create your BGA on the top layer of your design, the number one thing to remember – whether you are adding a JEDEC standard BGA with the BGA Generator command or reading in your pre-defined ball pattern from a text file or spreadsheet – is to flip it over.

BGA Generator, Page 1

BGA Text In, Step 4

Above, I’ve highlighted the options in the two main interfaces. Forget to check these and your PCB library part will be incorrect. Placing the ball pads themselves on the top layer is a much more obvious thing to do.

The next step is to add your die components. These will be mounted flip-chip, chip-up on the bottom surface of the layout. Your initial orientation is slightly less important this time, though, since with the die stack editor it is convenient to move the dies around and reposition them in the layout (whoever said there were no advantages to not BEING the substrate?).

Here, we have a flip-chip placed on the bottom. Note that the stack sits on layer bottom and has a substrate location of SUBSTRATE BOTTOM. These are both very important. In the member details chart at the bottom, you can see the orientation is correct at CHIP UP. The preview image in the upper-right corner always shows the substrate lowest in the ordering for consistency, but even here, the label clearly shows that this is the bottom of the stack-up.  

Now, as you route from the die bumps to the BGA balls, you will move up through the layers of the design. It might help to remember this if you name the layers to indicate this, with M1 being above the bottom and M5 (or whatever the count is) being immediately below the TOP layer.

Unlike a typical PCB layout tool, you can reverse the layer names of the outer layers so that TOP is at the bottom. This is something I would recommend against, however, unless you will be taking the time to set up all your mask layers and other derived or relational layers from the other classes in the appropriate order in the layer stack.

Flip Design View

A second option for this type of design doesn’t involve physically changing anything about your design. The BGA component may still be placed balls-down on the bottom substrate layer, with the flip-chip mounted chip-down on the top.

Using the flip design view, everything visually in the canvas is turned over, allowing you to see the substrate from the IC designer’s chip-up perspective. This can be done WITHOUT modifying any of your existing design flows or practices. No layer changes needed, no remapping of layer assignments or component orientations in the package design itself.

This can be particularly useful if you are just trying to communicate with the IC designer but still want to perform your own activities (routing, wire bonding, die stacking, etc.) in a manner consistent with what you would do for a design with a stacked memory module or similar.

You can perform any actions in the flipped view as you can in the standard view. The system will remap your cursor positions on your behalf. BUT there is one advisory I’m compelled to make: even when designing in flipped view, the cross-section layers are still in the same order. While it might look at though you’re moving from the die pads on the bottom to the balls on the top, in the cross-section, the layer order is that of a standard package substrate.  

Are Interposer Substrates Different?

Interposers add another interesting layer to things. These substrates are, as often as not, fabricated in silicon. This makes the desire to keep the same orientation as the chip even more natural. After all, you can consider the interposer as an extension to the chip substrates.

Ultimately, however, the driving factor in your decision will be your own. The interposer likely will mount to its own parent substrate, whether that’s a package or a board, with its die components on top (or, perhaps, on both sides!) if they are flip chips. That means that the interposer’s external interface is on the bottom as with your standard BGA package.

That means that whether you are closer with the chip designers with some influence on their bump pattern or with the board engineers planning the PCB fanout routing may be the deciding factor. Whichever it is, Allegro Package Designer will provide the opportunity to visualize and design in both orientations and to move between them. Your

What Flows Do You Follow?

Do you follow one of the methods above? Does your company have a custom flow for package layout, and does it have differences depending on the mounting method for the components? How do you adapt as your package gets more complicated, including both flip-chip and wire-bond components (maybe even some discrete and RF elements)?

Whatever steps you follow, make sure that, if you’re designing with your balls at the top of the cross-section, that your part orientation is right! When your package view is the right-hand image below in a balls-up layout and your PCB component isn’t the left-hand mirror image, there are problems over the horizon! So, be wise, be successful. Be the very best designer you can be!

(Should you ever find out late in the flow that you had an “oops” moment and need to reverse the X coordinates of all the pins to keep the instance pin mappings the same but correct the definition… use BGA text out the “mirror coordinates in y-axis” option then reimport the generated text file to correct the symbol definition without breaking the mappings).