PCB and Integrated Circuit Relationship in Board Design

An integrated circuit is a complete circuit formed on a piece of semiconductor, and a printed circuit board is the structure that connects several of them into a product. The relationship between the two is what decides most of the layout and much of the cost.

Two Different Technologies

The integrated circuit is built by adding and patterning layers on a silicon wafer, then separating the wafer into individual dies. Each die is placed in a package that protects it and fans its connections out to a usable pitch.

A printed circuit board is built by patterning copper on an insulating substrate and laminating several such layers together. Where the semiconductor process works at nanometres, the board works at tens of micrometres, and the two scales meet at the package.

What Each One Does

The integrated circuit provides the function: amplification, logic, memory, conversion or radio. It is small, it is fast, and it cannot be modified after it is manufactured.

The printed circuit board provides the connectivity, the power distribution, the mechanical support and the thermal path. It is large, it is slow by comparison, and it can be revised at any time by producing new artwork.

The Board as the Carrier

Almost every electronic product contains at least one board and at least one integrated circuit. The board holds the device, connects it to the other parts and provides the reference planes that its high speed interfaces need.

The board also defines the product mechanically. Connectors, mounting points and the outline are board features, which means the electrical and mechanical designs meet on the same sheet of data.

From Die to Package

The die is connected to the package by wire bonds or by a flip chip process, and the package presents the connections as pins, leads or a grid of solder balls. That transition is what makes the device usable on a board.

The package also sets the thermal path. A device in a large exposed pad package can conduct heat into the board, while a small plastic package with no pad relies almost entirely on the air around it.

From Package to Land Pattern

The land pattern on the board has to match the package dimensions within the tolerance of both. Pad length, pad width and the spacing between pads are calculated from the package drawing rather than copied from a library without checking.

A land pattern that is slightly too small produces opens and weak joints, while one that is too large produces bridging. Both are decided when the footprint is drawn rather than when the board is assembled.

Input Output Count Drives Layer Count

A device with many connections forces the board to provide somewhere for each of them to go. The input output count, the pitch and the arrangement of the balls decide how many rows can be escaped on each layer.

That calculation is what sets the layer count in most dense designs. It is also why the escape routing is planned before the stackup is fixed, rather than after the layout has been drawn and found impossible.

Power Delivery to the Device

A modern device draws a current that changes within nanoseconds, and the supply has to hold its voltage through that change. The path from the regulator to the die is a network of planes, vias and capacitors, and every element in it adds inductance.

This is where board and device design are most tightly coupled. The capacitor values recommended in a data sheet assume a certain loop inductance on the board, and a layout with a longer loop needs more capacitance to achieve the same result.

Reference Planes and Return Paths

Every high speed signal leaving a device has a return current that flows in the plane beneath it. The board provides that plane, and the continuity of the plane is what allows the interface to work at its rated speed.

Where a plane is split, the return current has to detour, which adds inductance and radiation. Signal integrity therefore depends on board features that are far from the signal trace itself.

Thermal Path from the Die

Heat leaves the die through the package and then through the board or the case. Copper area, thermal vias and the contact between the board and the enclosure are all part of the thermal design of the device.

The junction temperature is the number that matters, and it is calculated from the thermal resistance of the whole path. A board that provides copper beneath the package can change the answer by a large margin.

Passive Components Around the Device

Decoupling capacitors, pull up resistors, termination and filters are placed around the device and often define its performance. Their placement is a board decision, and the distance from the pin is part of the electrical specification.

Fewer external components is the direction of the industry, but the ones that remain become more critical. A single mis-placed decoupling capacitor can make the difference between a working interface and an intermittent one.

Test and Inspection

Fine pitch packages cannot be inspected visually, so their joints are checked by X-ray or by electrical test. The board has to provide the test access that makes those checks possible, which means pads and vias placed for the fixture.

Test coverage is designed with the product rather than added afterwards. The details of that stage are covered in our notes on X-ray inspection and in the assembly guidance on reflow quality.

How the Two Evolve Together

Each generation of devices adds connections and reduces pitch, and the board responds with more layers, smaller vias and finer lines. The materials follow, because the dielectric loss of a standard laminate becomes a limit at higher rates.

That progression is why a board designed for an older device cannot simply accept a newer one. The escape, the power delivery and the reference planes all have to be reconsidered.

Practical Consequences

Choose the package before the layout, plan the escape before the stackup, and treat power delivery and thermal path as part of the device selection rather than as board details.

Then document the assumptions. The stackup rules, the pad standards and the design checks used here are described in our notes on pad design and on layer stackup.

Process Control and Verification

On a design of this kind, package is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

FAQ

Is a printed circuit board an integrated circuit? No. An integrated circuit is formed on a semiconductor die, while a printed circuit board is a laminated structure of insulating material and copper that carries the devices and connects them.

Why does a device with more pins need more board layers? Because each connection has to leave the package through a via or a trace, and the number of rows that can be escaped on one layer is limited by the pitch and the line width.

Can the package be changed without changing the board? Only if the land pattern and the thermal requirements are the same. Devices in the same package family are usually interchangeable, but the pin functions must be checked.