Speakers
Description
Summary
Large scale leading edge experiments such as the LHC, invariably push the envelope
of standard electronics. The substrate or PCB is one of the most expensive single
component that is designed into such electronics. Coupled with the industrial scale
of production, it is inevitable that manufacturer and design team closely cooperate
and analyze the risks and trade-offs of aggressive (quick and dirty) design vs.
manufacturability.
Direct chip attach boards pose further unique challenges in terms of line pitch and
surface preparation. When coupled with flex and rigid-flex design, the complexity
increases exponentially, namely because of more processing steps, but also because
of the mechanical functions of the board. Linked with harsh environmental
conditions such as temperature, vacuum, radiation and vibration standard designs
will not live up to the requirements.
This paper will introduce briefly current and future circuit board materials and
their respective properties with respect to temperature, vacuum, humidity,
radiation and high frequency behaviour. The changes in dimension and absorption of
moisture of certain base materials, especially Polyimide, are to be taken into
consideration during design.
We will discuss the various stack-up options for rigid, flex and rigid-flex design
such as "classical", "bikini" and "book binder" and will touch on the trade-off
with respect to price and function. Moreover, the properties of coverfilm and the
issue of adhesive squeeze out are covered.
The proper design for mechanical requirements such as flatness, stiffness, bending
radius, dynamic stress and torque will be laid out. It will be shown how typical
designs may overlook stresses during assembly. Moreover, ceramic inlays may be a
useful method to combine the advantages of PCB technology with those of hybrids.
We will investigate the influence of temperature and its implication to the design
of the PCB. Special attention will be given to drilling and cleaning of blind vias
and plated through holes in inhomogeneous material stacks in order to prevent
barrel cracking and separation from the innerlayer during temperature stress.
Laser, plasma and mechanical drilling methods will be compared to each other. The
various root causes of delamination during manufacture, assembly and operation will
be analyzed and we will present the appropriate counter measures.
These include choice of material, matching copper weight with prepreg or adhesive
thickness and proper surface preparation.
Passivation of copper surfaces is important for solderability and bondability of
hybrid circuits. GS Praezisions AG will share their knowledge gained in application
over a ten year period. Common problems are poor bonding yield, Ni/Au corrosion,
Ni/Au cracks, probe marks on bond pads, poor wetting, and whisker building. Some
of these problems can be alleviated with improved designs while other will require
excellent manufacturing control.
In the advent of ever shrinking geometries, new technologies for art work imaging
are inevitable. High density lines need to be combined with ever shrinking vias,
posing new challenges for imaging, plating and etching. New technologies in each
field including their respective limitations are shown such as stacked vias and
laser direct imaging.
In conclusion, we would like to stress that an early cooperation with a competent
and experienced manufacturer may push the technological envelope while not
compromising on reliability and yield. Sufficient time must be allowed for design
review and validation.
