RELIABLE
INTERNAL
INTERCONNECTS
for
Magnetic
SMT Components
Pulse’s InterLock Base Technology Improves Density,
Consistency, and Resistance to Physical Stresses
©1997 Pulse Engineering, Inc. 1 G011.A (8/97)
EXECUTIVE SUMMARY
The Situation
As electronics manufacturing has steadily moved toward SMT processes and increased densities, the
issue of creating effective and reliable internal component interconnections has become a major
challenge. Such internal interconnections form the vital electrical pathway between the internal
component and the lead-frame, providing the external pins required for creating SMT solder
connections. Not only must these internal interconnections be extremely small to allow for ever-
increasing densities, they also must be designed to resist the physical stresses involved in the
component encapsulation process and the high temperatures involved in subsequent SMT solder
processes.
The semiconductor industry has addressed the internal interconnect challenge through the adoption
of highly automated micro-miniaturized bonding processes, such as welding and brazing. Because of
the essentially two dimensional nature of IC wafers, the automated micro-bonding machines are able
to precisely place and bond micro-thin wiring to both the IC die contact points and the corresponding
contact points on the lead-frame. The die is firmly locked into position in the lead-frame and the
consistently low profile of the wiring connection reduces any stress damage from occurring during
the component encapsulation process. Because the micro-wiring process uses a non-solder process
such as welding or brazing, the internal connection is also much less susceptible to damage from the
high temperatures that occur when the component is later subjected to reflow solder processes.
The Challenge
With larger three-dimensional components such as magnetic coils, the challenge of creating
effective, reliable internal connections is not quite so straightforward. Traditionally, magnetic
coils have been formed into SMT components in one of several ways. The first way is to mount the
coils onto a miniature PC board, solder their leads into place, solder the PC board to the lead-frame
pins, and then encapsulate the entire PC board into the SMT component. This method uses a
significant amount of hand labor operations, adding cost and time to the process and runs the risk of
creating solder joints with poor workmanship that can incur damage during the reflow process.
Another historic method consists of wire-wrapping the coils’ leads directly to the lead-frame pins,
hand soldering, and then encapsulating the entire part with the coils “floating” in place. Because the
flow of plastic encapsulation material can move the “floating coil,” there is the potential for undue
physical stress and/or damage to the wire-wrapped connection during the encapsulation process. Like
the PC board mounting method, the individual wire-wrapping method is also subject to the costs,
time delays, and inconsistencies inherent to any hand labor process. Dip soldering has also been
used. However, the coils have to be moved away from the termination area and then moved back into
position for encapsulation. Broken wire connectors and insulation damage can result.
©1997 Pulse Engineering, Inc. 2 G011.A (8/97)
The Solution: “InterLock Base”
As a leader in magnetics component technology, Pulse has developed a patented method for ensuring
the quality, consistency, and connection integrity of encapsulated coils and other three-dimensional
electronic SMT components. This solution, the InterLock Base, consists of an internal plastic carrier
that holds the coil firmly in place and provides precisely engineered “lead-channels” to lock together
the lead wires and the lead-frame pins. All of the InterLock Base interconnections are then
simultaneously dip soldered, providing for both efficiency and uniformity.
The Benefits
The benefits of using InterLock Base technology for the manufacture of SMT magnetic components
are multi-faceted. In general, the benefits can be grouped into three major categories:
Quality and Reliability Improvements
Improved solder joint reliability through parallel routing of coil lead wires and
lead frame pins
Protected interconnects for less physical stress
Protected coil assemblies during encapsulation process
Control of lead insulation “burn back” to improve Hi-Pot resistance
Manufacturing Process Efficiencies
Increased levels of mechanization in the component assembly process
Repeatable and consistent location of coils, coil leads, and lead lengths
Reduced use of manual placement and soldering operations
More uniform internal structure for more consistent encapsulation process, eliminating
voids and pin holes
Design Flexibility and Extensibility
Create components with multiple coils, chokes, etc. without using fragile wire-wrapped
“flying lead” interconnections or cumbersome “daisy-chain winding” of multiple
components
Changing coil/pin connections does not require a PC board change
Improved ability to efficiently position multiple components within a single encapsulated
package
Taken together, the accrued benefits of InterLock Base technology allow for the delivery of higher
density, higher quality, and higher functionality products to Pulse’s customers. The following
sections of this paper will discuss in detail the InterLock Base technology and how it contributes to
each of the above benefit areas.
©1997 Pulse Engineering, Inc. 3 G011.A (8/97)
How the InterLock Base Works
The primary objective of the InterLock Base is to facilitate miniaturization and to mechanically and
electrically join the magnetic component and the lead frame in such a way as to maximize
consistency and to minimize the risk of decoupling.
The InterLock Base is a preformed plastic component carrier that contains recessed “pockets”
designed for holding magnetic coils of various sizes and “channels” created to position the coils’
lead wires in firm contact with the lead frame pins. See Figure 1 for an example of how cores are
inserted into a three-pocket InterLock Base. See Figure 2 for an example of how the lead frame is
fitted to the InterLock Base. Figure 3 illustrates the completed transfer molded module.
The InterLock Base allows for physical isolation of the coils from the encapsulation, while at the
same time providing a mechanism for high density integration of multiple magnetic components
within a single SMT package. See Figure 4 below.
©1997 Pulse Engineering, Inc. 4 G011.A (8/97)
The steps involved in using the InterLock Base are as follows:
Step 1 Placement: The magnetic coils are placed into the InterLock Base pockets and held into
place using a silicon compound.
Step 2 Routing: The wire leads are routed into the appropriate channels, where they are held into
place by double-sided tape on the sides of the fixture below the InterLock Base. The
bottoms of the coil lead wires are then trimmed to the appropriate length to match the
length of the lead frame pins.
Step 3 Forming: The lead frame is preformed with right angle pins that fit the InterLock Base
channel grooves.
Step 4 Assembly: The lead frame is then fitted snugly into the InterLock Base channels and is
designed to exert spring tension to hold the lead wires firmly in place.
Step 5 Automated Soldering: The protruding leads of the assembled InterLock Base are then
subjected to a mass dip solder process that precisely wicks solder into the channels,
creating uniform solder joints between all coil leads and the lead frame.
QUALITY AND RELIABILITY BENEFITS
Improved Solder Joint Reliability
The InterLock Base routes the coil lead wires and the lead frame pins in a uniformly parallel
configuration for soldering, creating solder joints that are significantly more consistent and reliable
than hand soldered connections. The natural capillary action of the molten solder wicks the solder
joint into the InterLock Base’s lead wire channel, just as solder is wicked into a plated via hole on a
printed circuit board. The completed solder joints are electrically sound, consistently uniform, and
highly resistant to physical stresses. Figure 5 illustrates the InterLock Base soldering process where
multiple assemblies are dip soldered in a mass production process.
Protected Interconnects for Less Physical Stress
Because the final solder joints are mechanically captured in the InterLock Base lead channels, they
are much more protected from physical stresses than traditional hand-soldered joints which are either
exposed on the surface of the internal PC mounting board or are merely wire-wrapped to the lead
frame pin. In both of these cases, the stresses associated with the flow of encapsulant materials and
the subsequent temperature spikes of the SMT reflow solder process will “pull” on the lead wires
and can damage or break the solder connection. In contrast, the InterLock Base and its captured
solder joints will react to such physical stresses as a single unit, thereby eliminating any “pull” effect
on the joints themselves.
©1997 Pulse Engineering, Inc. 5 G011.A (8/97)
Protected Coil Assemblies During Encapsulation Process
In addition to protecting the solder joints, the InterLock Base's structure also protects the coils
themselves from damage during the encapsulation process. Held firmly in place within the InterLock
Base's recessed pockets, the magnetic coils are not subjected to any unplanned movements caused by
the encapsulant flow. This reduces physical stresses on the coil and its lead wires which can
otherwise cause component damage and/or failure.
Isolation of Lead “Burn Back” to Improve Hi-Pot Resistance
With traditional encapsulated coils, the tapered “neck-down” area at the end of the insulation burn-
back is typically the weakest electrical point of the assembly. If the insulation has been burnt back
too close to the coil during the hand soldering/assembly process, there is a high likelihood for arcing
directly to the coil during Hi-Pot testing, creating a failure. Because the InterLock Base process
isolates the burn-back so that it is always within the wiring channel, the risk of shorting to the coil is
completely eliminated.
Manufacturing Process Efficiencies
The use of InterLock Base technology allows for significantly increased levels of efficiency and
effectiveness in the component assembly process. The elimination of manual soldering results in a
more repeatable and consistent end product. Because the InterLock Base provides a uniform
mechanical structure that precisely positions the magnetic coils, lead wires, and lead frame pins in a
consistent relationship to each other, subsequent processes such as soldering can be conducted on a
mass production basis. This highly controlled physical configuration also allows for a more uniform
and consistent encapsulation process.
Another benefit of such a mechanized and highly structured production process is that higher quality
output levels are easier to maintain. By improving the consistency and uniformity of the overall
production process, resources can be refocused from inspecting variations in the output of
individuals and used for high-level process control activities, such as Statistical Process Control.
The benefits of process improvements in both efficiency and quality accrue directly to Pulse’s
customers in the form of industry-leading cost competitiveness and product reliability. As pin count,
package size, and the number of internal coils increase, the advantage in reliability and costs of the
InterLock Base becomes more significant.
Design Flexibility and Extensibility
In addition to improving the quality, reliability and consistency of Pulse’s SMT magnetic
components and the manufacturing processes used to produce them, the InterLock Base technology
also allows Pulse to better serve its customers and markets by streamlining the process for
introducing and refining new higher integration product designs.
While today’s miniaturized telecommunications applications can greatly benefit from the single
package integration of multiple magnetics (e.g. transformers, chokes, etc.), traditional packaging
methods have not been conducive to such integration. Without the use of InterLock Base technology,
multiple devices must be connected either by wire-wrapping their leads together in a series of “flying
lead” connections, or using a single length of coil wire to wind multiple components in a continuous
©1997 Pulse Engineering, Inc. 6 G011.A (8/97)
“daisy chain.” The “flying lead” method is highly unreliable because of the stresses that are placed
on these multiple unprotected connections during encapsulation and/or SMT reflow. On the other
hand, the “daisy chain” method significantly increases production costs because subsequent coils
using the same strand of continuous wiring must be individually hand-wound, thereby sacrificing all
the benefits of modern machine winding methods.
The InterLock Base offers a proven, straightforward alternative method for efficiently positioning
and connecting multiple components within a single encapsulated package. Because the InterLock
Base is a pre-formed plastic carrier, it can be designed in many different physical configurations.
Standardized bases with pockets for various sizes of coils can be used for quickly prototyping new
component combinations or specially structured packages can be developed for non-standard
component sizes.
Just as the InterLock Base wiring channels can be used to match coil wiring to lead frames, they can
also be used effectively to interconnect the leads between multiple devices within the same SMT
package. The leads from different internal components are simply routed into a common channel on
the InterLock Base and are then dip soldered together in the same pass with the lead frame pins.
The InterLock Base enables Pulse to quickly respond to customers’ requirements for increasingly
higher levels of integration and flexibility. It is especially beneficial to customers in the networking
and telecommunications arenas who must match their products to constantly evolving standards,
frequency spectrums, and market demands. Pulse continues to develop the InterLock Base
technology to support non-magnetic components and more complex electrical circuits.
Summary
The increasing use of SMT production technology and the market demands for even smaller
miniaturized products is constantly driving the need for higher component densities. An inherent
challenge in achieving these densities involves the development of effective internal interconnection
methods that provide the pathway between the encapsulated component and the “outside world.”
The SMT magnetics industry has had to deal with the additional challenges of positioning large,
three-dimensional components and their lead wires and then holding them in place during the
soldering and encapsulation processes.
Pulse has responded to this challenge with the development of a patented InterLock Base technology
that provides for an efficient, uniform, mechanized method of achieving consistent high quality
interconnection results. In addition, InterLock Base technology provides a flexible and extensible
design foundation for responding to the higher level integration needs of Pulse’s customers.
Bibliography:
U.S. Patent for “Electronic Micro-Miniature Packaging and Method,” granted May 14, 1991.
©1997 Pulse Engineering, Inc. 7 G011.A (8/97)
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©1997 Pulse Engineering, Inc. 8 G011.A (8/97)
