EXECUTIVE SUMMARY
 
This deliverable report is attached to the demonstrator deliverable 1.3, as a result of task 1.4.2: demonstrating the mechanical packaging of the strain sensor and its read-out electronics, ready to be mounted on the rebar.
The work presented in this deliverable is strongly entangled with the development of the strain sensor (task 1.3.3), the development of the readout electronics ASIC, the development of the readout electronics circuitry (task 1.4.3).

For this deliverable the main challenge was found in finding a package that can reliably withstand the strain subjected to the strain sensor (max. 3%). Two solutions are found:
1) The first solution is valid for circuits which have a similar length and width (square-like surface of the PCB). On these samples the strain can be countered by a thicker PDMS layer on top and below the circuitry. A flex interconnect is used to bridge the strain-sensor to PCB gap, which are positioned at a different height relative to the rebar: the strain sensor is glued to the rebar, while the electronics are supported by a thin layer of PDMS. The PCB is a standard PCB and therefore cost-effective, the amount of PDMS used is less than in the second solution. The SMD-components are however subjected to a shear force, which might introduce reliability issues.

Figure 1 : Overview of the first packaging scheme proposed

2) The second solution is to be applied if the PCB for the sensor and the readout electronics has a rectangular form factor, i.e. one dimension is larger than the two other dimensions. This is the case if the width of the sensor node is restricted by the end users. In this case a 3% elongation will destroy both the soldered interconnections between the SMD-component, as well as the copper lines on the PCB. The solution consists in folding the PCB by implementing flexible regions, while the rigid components are mounted on rigid PCB-parts. This solution uses a complex molding step using PDMS.
This solution is also applicable for circuits with a square form factor, further decreasing the stresses on the soldered joints.

Figure 2 : Overview of the second packaging scheme, smaller in length.

With this deliverable a proof of concept of both mechanical demonstrators is given, with a special focus on the second solution. The choice in between both solutions will depend on :
1) The end-user requirements : maximum width of the strain sensor package
2) Reliability tests of the package when subjected to repetitive 3% strain elongation.
3) The form factor of the ASIC : SOIC package or wire-bonded die
The answer to question 3 will depend on 1 and 2: a SOIC package is not compatible with a rectangular form factor of the PCB. A wire-bonded die is however more expensive in fabrication, and introduces extra reliability issues.