When designing multilayer PCBs it is recommended to balance the copper area of internal and external layers. This should be done by the use of a cross hatch rather than a solid copper plane. When using a cross hatch ensure that the track width used is greater than 6 thou. A preferred width is 10 thou.
Specifying tight track width/spacing simply decreases the yield from a panel and raises costs. We recommend that you use 8 thou (or higher) track/space but in general the larger the track/space, the higher the yield and the lower the cost. On Multilayer boards, a clearance from internal pad edge to the internal copper plane of 10 thou is required. This will almost certainly avoid the chance of internal shorting or breakout.