Thermally stable pagewidth printhead assembly

1. A pagewidth printhead assembly comprising:

an elongate support beam;

an elongate shell at least partially enclosing and restraining the support beam; and

a plurality of printhead integrated circuits mounted to the support beam so as to be substantially aligned with one another along said pagewidth,

wherein the shell has an effective coefficient of thermal expansion substantially equal to that of the plurality of printhead integrated circuits.

2. A printhead assembly according to claim 1 , wherein the support beam has formed therein at least one printing fluid reservoir arranged in fluid communication with the plurality of printhead integrated circuits.

3. A printhead assembly according to claim 1 , wherein the shell is a laminated structure having an odd number of laminae arranged so that the outer laminae have the same coefficient of thermal expansion as one another and a different coefficient of thermal expansion to that of the inner laminae.

4. A printhead assembly according to claim 3 , wherein the coefficient of thermal expansion of each laminae is different than that of the printhead integrated circuits.

5. A printhead assembly according to claim 3 , wherein the outer and inner laminae are formed of different metals.

6. A printhead assembly according to claim 5 , wherein the outer laminae as formed of invar.

7. A printhead assembly according to claim 1 , wherein the printhead integrated circuits are fabricated from silicon and constructed using microelectromechanical techniques.

8. A printhead assembly according to claim 7 , wherein each printhead integrated circuit comprises a plurality of microelectromechanical devices, each device comprising a printing fluid nozzle, nozzle chamber and actuator for ejecting printing fluid onto print media.