 The
purpose of the drilled through-hole is to make an opening
through the printed circuit board that:
|
 |
permits
subsequent processes to make an electrical connection
between top, bottom, and all internal pathways, and |
|
allows
through-hole components to be located precisely and
mounted with structural integrity. |
The
quality of a through-hole drilled in a circuit board is
measured by its ability to support and accept the plating
and soldering operations required to form a highly reliable,
non-degrading electrical and mechanical connection. When
circuit boards had traces on only one side, the quality
of the drilled hole was not very important. Later as double-sided
boards with plated through-holes became common, drilled
hole quality had to improve. Since today's multilayer
boards require connections to the inner layers as well
as the surface pads, the quality of the drilled hole is
paramount in ensuring reliable connections. |
| Although
it is painful to abandon familiar practices, we must
recognize when they no longer meet our needs. The circuit
board fabrication industry is experiencing rapid technological
change. The driving force behind these changes has been
the increased use of surface mount technology (SMT)
and the consequent need for designers to maximize the
use of board real estate. Consequently, the industry
has experienced increases in the number of holes per
square inch, smaller SMT pads, conventionally drilled
vias as small as .0039", increased layer counts, tighter
annular rings, as well as blind and buried vias. There
is no manufacturing area where these changes have had
more of an impact than in the drill room. |
|
James Block, President of Laminating Company of America
(LCOA), states that the root cause for as many as 85
percent of all circuit board failures can be traced
back to drilling (1). When the entire board manufacturing
process is examined, it becomes apparent that many post-drilling
operations are corrective measures designed to overcome
shortcomings in the drilling process. For example, the
use of mechanical scrubbing to remove burrs, chemicals
to remove resin smear and bonded debris, etch-back to
expose glass fibers, and acid or alkaline cleaners to
remove contaminants all are methods for addressing problems
that result from the drilling process. Logic tells us
that at some point it no longer makes sense to compensate
for drilling problems, and that we should shift our
focus to address the source of the problems. |
| Generic
drilling practices are no longer acceptable. Specific
drilling processes must be developed and validated for
each unique type of board technology. A key concern
in the change of mindset from applying generic drilling
practices to tailoring a specific and unique process
for each technology is the selection of consumables
used in the drill room. Historically, cost alone determined
the selection of supplies for the drill room. However,
as technology continues to push the drill room for improvements,
the role of consumables can no longer be overlooked.
To survive, circuit board manufacturers have had to
develop a unique set of processing parameters for each
type of printed circuit board. As is often the case
when conventional thinking is challenged, innovative
fabricators have been rewarded with greater efficiencies,
improved yields and reduced costs.
|
| Although
there are many items on the consumables list in the
drill room, the three most important are: |
|
drill
bits, |
|
entry
materials and |
|
backup
materials. |
 |
| Choosing
Drill Bits |
| The
carbide drill bit is the most critical of these three
consumables. Drill bit manufacturers have developed
numerous styles and series of drill bits to help support
the varied applications in today's board manufacturing
environment. Flute length, web thickness, point geometry,
and back taper all need to be considered when selecting
the right drill bit for the application. |
| Mr.
Tech Dweeb Tech Tip |
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Today's
typical basic drill geometry incorporates
a 15 degree primary angle, a 30 degree
secondary angle, a high helix, polished
flutes, relieved margins, a back tapered
body , and a fine grain tungsten carbide
base. |
|
|
| The
minimum flute length must equal the total drilled hole
depth plus at least .050" of unused drill flute. (The
drilled hole depth is the sum of the total laminate
thickness, the entry material thickness, and the exit
material penetration.) The unused flute measurement
is made above the stack while at the bottom of the drill
stroke. This extra flute length is necessary to allow
debris to be evacuated by the vacuum system. Failure
to remove debris from drill flutes can result in degraded
hole quality and even drill bit breakage. |
| To
keep drill bits sharp and to avoid breakage, they are
generally used for 750 to 1,500 hits on multilayer circuit
boards and for 2,000 to 3,000 hits on double-sided boards.
Hit counts greater than 3,000 can be realized on single-sided
boards. |
| Drill
bits can be repointed from 1 to 5 times depending on
the diameter of the bit. Typically from .002" to .005"
is removed by grinding during the repointing process.
The smaller the bit, the fewer times it can be repointed
since smaller diameter holes are more critical and require
superior drilled hole quality. |
| Drill
bit replacement and repointing represent substantial
expenditures for circuit board manufacturers. Depending
on the diameter and style of the drill bit, average
prices range from just over $1.00 to more than $20.00
per bit. Therefore proper storage, handling, and inspection
are critical to ensure maximum life span and optimum
performance, and to contain costs.
|
Drill Bit Geometry
Based on a figure in:
Vandervelde, Hans. PCB Handbook. McGraw-Hill,
2001.
|
|
|
| Determining
the Best Entry Material |
| The
second most important drilling consumable is the entry
material. The main purpose of the entry material is
to prevent drill breakage by centering the drill bit.
In addition, the entry material helps avoid copper burrs,
reduce contamination in the hole and on the drill bit,
and prevent pressure foot marks from the drilling machine. |
|
Many types and thickness of entry material are available
on the market today. Aluminum composite, solid aluminum,
melamine products, and aluminum-clad phenolics are the
most common. |
| Selecting
the right entry material requires a thorough understanding
of the drilling application for which it is to be used.
Although aluminum composites are typically the most
expensive, they do a tremendous job improving accuracy
and dissipating heat. In addition, they leave no hole
contamination. Phenolic materials are less expensive,
but often warp and can contaminate the hole wall, possibly
resulting in problems during subsequent processes. Solid
aluminum provides good burr suppression and no contamination,
but increases the risk of drill bit breakage for small
diameter bits. |
| A
lot of work is being done to develop a new generation
of lubricated entry material to help further reduce
heat generated during the drilling process. Heat generated
during the drilling process destroys the optimal condition
of the hole wall. When a hot drill bit is extracted
from a hole, there is a risk of smearing melted resin
over the inner layer attachment pads. Although the amount
of heat generated during drilling can be minimized by
controlling infeed rates and drill speed, the selection
of proper supplies for the application is essential.
|
| Proper
selection of entry material for a particular drilling
application is very important due to the wide range
of cost for different materials. Costs can range from
33 cents per square foot for a phenolic material to
around $4.00 per square foot for a specialty product.
|
|
| Selecting
the Appropriate Backup Material |
| The
third most important drilling consumable is the backup
material. The purpose of the backup material is to prevent
exit copper burrs on the underside of the drilled stack
and to provide adequate space for drill stroke termination.
An acceptable backup material does not contaminate the
hole and helps cool the drill bit, thereby improving
hole quality. |
|
There is a great variety of backup materials available
on the market today. Selecting the appropriate "backer"
requires extensive testing and qualification since few
of the products marketed as backup material were engineered
specifically for circuit board drilling. Typical materials
include aluminum-clad wood core composites, melamine-clad
wood core composites, solid phenolics, and even paper-resin
hard board. |
| The
backer deemed appropriate for a particular drilling
process must also have a tight thickness and flatness
tolerances. It should contain no abrasives that would
increase drill wear or contaminates that could be evacuated
through the drilled hole. The surface should be smooth
and hard to properly suppress exit burrs. |
| As
with the choice of entry material, it is important to
match the backup material to the application since the
cost of backup materials ranges from about 25 cents
per square foot for hard board to around $6.00 per square
foot for a specially engineered, lubricated backer. |
|
| Finding
the Right Combination |
| The
great variety of materials available and the wide range
of their cost create a multi-dimensional matrix of possible
solutions to the dilemma of choosing the optimal combination
of drill bits, entry material and backup material for
a particular application. Proper selection can be made
only through continuous testing, a thorough understanding
of the technology employed in the application, and an
unswerving commitment to quality. The drill room must
keep up with the processing requirements of today's
circuit boards, and keep in touch with customers' demands
regarding quality, technology, and cost. |
| Future
articles in the dsi Quarterly
Tech Review will examine other variables that affect
the quality and cost of the drilling process. |
| Sources: |
| 1. |
Vandervelde,
Hans. PCB Handbook. McGraw-Hill, 2001. |
| 2. |
Goulet,
David. Bare Board Drilling: Trends and Developments
in Printed Circuit Board Drilling. Miller Freeman Books,
1992. |
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