 The purpose of the drilled through-hole is to make an opening
through the printed circuit board that:
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permits subsequent processes to make an electrical
connection between top, bottom, and all internal pathways, and |
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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.
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| 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.
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| Although there are many items on the consumables list
in the drill room, the three most important are:
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drill bits, |
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entry materials and |
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backup materials. |
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| 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
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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. |
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| 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.
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| 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.
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| 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.
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Drill Bit Geometry
Based on a figure in: Vandervelde, Hans. PCB
Handbook. McGraw-Hill, 2001.
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| 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. |
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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.
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| 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.
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| 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.
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| 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.
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| 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. |
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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.
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| 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.
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| 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. |
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| 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: |
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1. |
Vandervelde, Hans. PCB Handbook. McGraw-Hill,
2001. |
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2. |
Goulet, David. Bare Board Drilling: Trends
and Developments in Printed Circuit Board Drilling. Miller Freeman Books,
1992. |
