Camera Module PCB: Design and Manufacturing

The Board Inside a Camera Module

A camera module is a stack of precision parts: the lens, the holder, the actuator that moves the lens for focus or stabilisation, the image sensor, the filter and the printed circuit board that carries the sensor and connects it to the rest of the product. Everything except the optics is mounted on the board, and the board therefore carries the mechanical tolerances of the whole module as well as its electrical function.

Two competing requirements shape the design. The board has to hold the sensor exactly in the optical axis and at the right distance from the lens, which makes flatness and dimensional stability critical. And it has to carry a high speed serial interface and a set of analogue supplies into a very small space, which makes the layout dense and the signal integrity demanding.

Constructions in Use

• Rigid board with a connector. The simplest construction, used where there is space behind the optics. The sensor sits on the rigid board and a board-to-board connector links it to the main board.
• Rigid-flex. A small rigid section behind the sensor, a flexible tail that carries the interface to the connector, and a stiffener behind the sensor to keep it flat. This is the dominant construction in phones and tablets because it fits around the battery and the other mechanical parts.
• Flex with a stiffened sensor area. A fully flexible circuit with a bonded stiffener under the sensor, used in the smallest modules and in endoscopes.
• Ceramic or glass substrate. Used in the highest performance sensors and in some automotive modules, where the substrate under the sensor has to have a very low expansion and a very fine pitch to connect the die.

Our notes on flex PCB assembly describe the handling and support that flexible and rigid-flex camera boards need.

Connecting the Sensor

The image sensor is an area array device, and its connections are on the underside, so they cannot be soldered with an iron and cannot be inspected optically. Two approaches are used. A wire bonded sensor has the die mounted on a substrate and connected with fine wires, which is a packaging step performed by the sensor supplier. A chip scale package has the connections as an array of solder balls or copper pillars, which the module maker reflows onto the board.

Whichever is used, the joint is invisible and the consequences of a poor one are subtle: a partially open connection produces an intermittent column or row of pixels rather than an obvious failure. X-ray with automated analysis is the standard inspection, and a thermal cycling test on a sample confirms that the joints survive the temperature range of the product. Our notes on PCBA testing describe how the inspection is carried out.

Signal Integrity

The interface between the sensor and the processor is a high speed differential serial link, and it is the part of the design that decides whether the image is clean. Three rules matter more than the rest.

Keep the pair together. The two traces of a differential pair run at a constant spacing, with matched lengths, over a continuous reference plane. Anywhere the pair separates, or crosses a plane split, the impedance changes and the link loses margin.

Keep the skew small. The length difference between the two traces of a pair is controlled to a fraction of a millimetre. A mismatch turns part of the differential signal into a common mode signal, which radiates and reduces the eye height.

Keep the reference continuous. The plane under the pair must be unbroken from the sensor to the connector. Where the pair has to change layers, provide a return via next to the signal via.

The analogue supplies to the sensor also matter. They are low noise, low current rails that feed a sensitive analogue circuit, so each is filtered close to the sensor and routed away from the switching supplies and the serial pairs. Our notes on PCB design and layout cover the routing practices that deliver those conditions.

Mechanical Precision

The optical axis is set by the sensor position, and the sensor position is set by the board. Two properties therefore have to be controlled.

Flatness. The sensor must sit in a plane parallel to the lens. A board that is bowed, or a stiffener that is not flat, tilts the sensor and produces a focus error that varies across the image.

Dimensional stability. The board expands with temperature and absorbs moisture, and a rigid-flex board with a polyimide tail moves more than a rigid one. The sensor area is stiffened to reduce that movement, and the module is designed so that the focus adjustment can absorb the remainder.

The board thickness tolerance also matters where the sensor height is set by a stack of parts. A tight thickness tolerance on the rigid section is cheaper than a mechanical adjustment in the module, and it is usually the first thing specified.

Thermal Behaviour

A camera sensor warms as it operates, and both the dark current and the focus position change with temperature. Where the module is in continuous use, such as in a security camera or an automotive camera, the board is designed to spread the heat from the sensor and from the processor evenly rather than to concentrate it.

In an automotive module the situation is reversed in winter, and the opposite problem appears: a lens that is at a different temperature from the sensor holder will defocus as it equalises. The mechanical design and the board stiffness both help, and the assembly is tested over the full temperature range. Our notes on PCB manufacturing describe how the material properties that control this are specified and verified.

Assembly and Test

Assembly is a precision process. Solder paste is printed, the sensor is placed with a machine whose accuracy is a fraction of the pad pitch, and the assembly is reflowed with a profile chosen for the sensor package and for the flexible tail. The lens holder, the actuator and the filter are attached afterwards, and the module is focused and the alignment is set, often with a mechanical adjustment that is cured in place.

Test is done at several stages. A functional test on the assembled module confirms that the interface works and that the image is present and free of defects. A pixel test checks for dead and hot pixels, which the processor can sometimes mask, and a focus test confirms the modulation transfer function at several points in the field. Where the module includes a focus actuator or a stabiliser, the actuator is exercised and its response is measured. Our notes on PCB assembly describe the surface mount process the module shares with other fine pitch assemblies.

What It Costs

A camera module board is expensive relative to its size because of its geometry and its tolerances rather than its material. A small rigid-flex camera board in prototype quantity commonly costs tens of dollars per piece, and the price falls with volume but less sharply than for an ordinary board, because the rigid-flex construction, the fine line width and the flatness requirement all consume plant capacity.

The comparison that matters is against the module. The board is a small part of the cost of a camera module, and the cost of a failed module, including the expensive sensor, is far greater than the difference between a good board and a cheap one. That is the reason the laminate, the stiffener and the thickness tolerance are specified tightly and left alone.

Design Practice

• Fix the stack with the sensor datasheet open. The pad geometry, the stiffener position and the keep-out areas around the optical path come from the sensor supplier, not from the layout.
• Keep the differential pairs short and matched. Route them before anything else, on the layer with the cleanest reference plane.
• Filter the analogue supplies at the sensor. One small filter per rail, placed close to the pin, with the return to the sensor ground.
• Stiffen the sensor area. A bonded stiffener under the sensor reduces the flatness and the thermal movement, and it also protects the sensor during assembly.
• Design the tail for handling. Keep the bend radius generous, avoid vias in the bend area and add a stiffener behind the connector.
• Plan the test points. Bring the interface and the supply rails to pads, because a finished module is difficult to probe.

FAQ

Why is a camera module PCB often rigid-flex? Because the sensor needs a flat rigid area and the product needs a flexible tail to reach the processor around the other mechanical parts.

Can a camera board be built on ordinary FR-4? Yes, where there is space behind the optics for a rigid board and a connector, as in many industrial and automotive modules.

What limits the image quality on the board side? The signal integrity of the sensor interface, the quality of the analogue supplies, and the mechanical flatness of the sensor mounting.

How is the sensor joint inspected? With X-ray and automated analysis, because the connections are underneath the package and cannot be seen optically.

Conclusion

A camera module board carries the optical alignment and the sensor interface at the same time, so it is a mechanical part as much as an electrical one. Route the high speed pairs first and keep their reference continuous, filter the analogue supplies at the sensor, stiffen the sensor area for flatness and thermal stability, and inspect the invisible joints with X-ray. The board is a small part of the module cost and a large part of the module risk.