5.1 Manufacturing Procedure - Part 1
Fabrication of MIBs utilizes well-established thick film
screen-print, dry, fire operations. To manufacture high reliability boards
requires the careful application of screen printing basics and good process
control. With a well- controlled process, yields can be very high. For
example, with in-process rework, yields on 3 x 5-inch, 6-conductor-layer MIBs
approach 90%, although initial yields are generally 30% to 40%, reach 60%
within 3 to 6 months, and maturity after 1 1/2 years.
Formal operator training and certification are desirable. The
extent of training will vary with each facility depending on the level of
Because of equipment differences, each
facility must optimize its process. A general process flow diagram is shown as
5-1. It can be
used with either copper or noble metal with the restrictions that, excepting
the air-fire cleaning step, copper requires a nitrogen furnace atmosphere, and
noble metal requires a solderable material for the top via fill and solder pad
In-process inspections occur after each
printing. Although Figure 5-1
shows only one inspection after drying, actually there is a quick
scan by the operator at printing, another (more thorough) after drying which
includes touchup, and a third, possibly a formal QC inspection, after firing.
Formal QC inspection during processing, when applied, is normally used only
for the conductor.
There are arguments that the formal inspection needlessly adds
handling and interferes with the production flow while increasing cost.
Inspection during the build is extremely important. When and who inspects, in
most cases, depends on manufacturing volume. The printer operator performs
most of the inspection and touchup operation when the lots are small (less
than 50 pieces) and the production demands of rate are not excessive. Large
lots and high production rates by necessity remove the printer operator from
the in-process inspection task. Although formal QC inspection of conductors
occurs mostly under these conditions, in-process computer controlled pattern
recognition inspection would suffice for either.
Typical criteria for inspection are contained
in Figure 5-2. The criteria presented dwell heavily on conductors; however,
dielectric printing and especially via filling are very important. Via filling
is shown after each dielectric print in Figure 5-1
. In some processes, however, it occurs after
two or more dielectric prints.
Artwork is often used as an inspection aid for via holes in
dielectric and for via fills. Visual inspection, using a stepping program and
a video presentation, has proved effective.
In-process electrical testing is used to check for interlayer
shorts where voltage and ground planes are directly above each other. Full
probing of inner layers is costly and, for nearly all applications,
In-process corrections (touchup) are necessary and important,
albeit many times hidden steps in the process. Considering the area of the
MIBs, the number of layers, the high via count, and the fineness of lines,
yields would be seriously affected without touchup. Both conductors and
dielectrics are touched up, the amount reflecting on the condition of both
paste and screens.
Conductor touchup generally requires only a minor addition of
paste. If the conductor layer has many defects caused by a bad screen or poor
printing control, it should be washed off prior to firing. Occasionally, a
conductor layer is reprinted over the original (after firing). This procedure
is viewed with apprehension when copper conductors are used. At present, it is
suspected that double printing copper may be associated with a failure
mechanism typified by increased grain size and cracking at the grain
boundaries during temperature cycling.
Touchup of a dielectric layer usually
consists of cleaning up a clogged via. Because of the large area and the
multiplicity of defects caused by a poor print, dielectric is seldom applied
by hand to repair holes--it is better to wash off and reprint. If a problem is
relatively minor, a new screen or change in viscosity for subsequent prints
should be sufficient since the multiple prints used to form each layer provide
a degree of repair. If a problem is found after the final print of the layer
build-up, it would be possible to add another print as added insurance--if
there is no critical thickness constraint. Figure 5-1
shows three prints per layer, although four
prints are used by some manufacturers.
Via fill problems are seldom touched up by hand considering the
large number of vias per layer. It is better to inspect and wash off the wet
print if there are faults. If vias are under-filled, a double wet pass or a
second print can be used. Most via fill problems are related to the fill
screen or viscosity. Uneven filling is generally the result of an improperly
developed dielectric screen giving a large via hole and the corresponding via
fill screen providing too small a via fill.
PROCESSING ELEMENTS & CONTROLS
A thick film MIB production line requires screens, a large
well-controlled printer, a dryer, and a conveyor furnace. Most of the
equipment for thick film processing has been developed over a period of 20
years and generally has sufficient controls, including printer-squeegee speed,
pressure, and breakaway; furnace-belt speed; temperature controls; and gas
It is incumbent upon the MIB manufacturer to
properly define each procedure with a process control document. Table 5-1
is a listing of
production controls for process monitoring.
5-1. PRODUCTION CONTROLS
At fabrication and before each
Measure and adjust before each
Each set up and on a sample basis
Log each day
Firing sequence*, time between different
(conductor, dielectric), belt spacing
Numbered substrates that reflect firing sequence can be
process analysis, especially during initial
Screen Printing Area
A clean, dust-free, especially lint-free environment is
desirable, if not absolutely necessary, for the production of MIBs. Most areas
are Class 100,000 or better. As a minimum, all areas should use smocks and
tack mats. Bonnets for hair covering and booties for shoes are also used in
many facilities. The problem of lint is especially troublesome in nitrogen
firing. The lint discolors the dielectric, leaving a carbon trace which may
require ' picking" to remove, depending on size and placement.
Particulate contamination also occurs in the furnace from belt
debris and driven-off paste components, carbon, and oxides of lead and
bismuth. Periodic cleaning is necessary to control furnace contaminants, the
frequency of which, depending on the production rate, ranges from weekly to