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HIGH SPEED DISPENSING OF SURFACE MOUNT ADHESIVE BETWEEN SOLDER PASTED PADS

Introduction

Surface mount
adhesive has been a part of electronics manufacturing applications from
the beginning of SMT. It has been used, in conjunction with wave
soldering processes, to successfully solder millions of components to
the bottom sides of printed circuit boards. In an effort to make the
manufacturing processes more robust and to improve the quality of the
assemblies, a solder paste printing step and a reflow soldering step
have been added to many traditional bottom side assembly lines. These
operations are added in order to decrease defects such as missing
components and insufficient solder joints. Both SMT (double sided
reflow) and Through hole (mixed SMT/THT) processes can benefit from this
process utilizing adhesive and solder paste. Some of the process
considerations are nozzle design, pad design, PCB layout, stencil
design, and adhesive properties. This article will deal with the
characteristics that must be considered in setting up this process, how
it can be implemented successfully, and typical line configurations
associated with this process. The major foundation of traditional
bottom side assembly processes is the adhesive.

Adhesive Selection

 When selecting an
adhesive for applications involving the dispensing of surface mount
adhesives between solder pasted pads, it is important to choose an
adhesive that is formulated to give very specific rheological, or flow
properties. The adhesive selected should be formulated to allow for a
higher profile dot that exhibits very little slump. This will allow the
glue to contact the component, above the height of the solder paste
deposition, when the component is placed. Dots dispensed for this type
of application should have a tall, cylindrical shape as opposed to the
typical triangular Hershey kiss dot profile. The typical profile may
not allow the glue to properly adhere to the component prior to curing
and then hold the component through wave soldering. This will cause a
large number of missing component errors to be seen following the wave
soldering operation. Excessive missing components following manual
assembly may also be seen because the glue joint is not large enough to
provide the strength needed to hold the components in place.

 The surface mount
adhesive chosen for these applications must also have a high green
strength in order to hold the component prior to the curing process. It
is this green strength that also helps the adhesive to maintain the
tall cylindrical dot shape needed when dispensing between solder pasted
pads. Without it the adhesive deposit will slump, losing contact area
with the component, and causing a decrease in the strength of the
adhesive joints.

 In adhesive
dispensing processes utilizing heat, it is difficult to achieve the
necessary dot height. By applying heat to the adhesive, the material’s
viscosity is lowered, allowing it to flow more easily. This type of
flow characteristic will cause the adhesive dot to slump after
dispensing. Problems related to the adhesive not contacting the
component (missing components after wave soldering, etc.) will increase
in frequency, as well as the number of opportunities for defects such as
pad contamination to occur.

Board Design

 Typically,
surface mount component pads are designed for either adhesive deposition
or the screening of solderpaste. The pad spacing is generally smaller
for solderpaste application as opposed to that of adhesive deposition.
For example, the component pad spacing between the pads of a 0603 chip
cap/resistor is typically 0.020”, if the board was designed to be screen
printed with solderpaste. The pad spacing for the same board can be
0.040” if adhesive deposition was to be utilized. A 0.030” diameter dot
of adhesive would easily be recommended for use if the component pads
on the board were indeed designed for adhesive deposition. However, if
the pad design for the same board was originally designed for
utilization of solderpaste, as a method of adhering the component to the
board, obviously, an 0.030” diameter dot of adhesive would be too
large, as the spacing between the pads is now 0.020”. A 0.015” to 0.018”
diameter dot is required for this particular application.

 In designing pad
spacing and component spacing, the height of the pad and the solder
paste deposition must also be taken into consideration. Typically, the
height of an adhesive dot is one half the diameter of the dot.
Depending upon the material used for the pads, it would be possible
design a board which would be impossible to print and dispense adhesive
on. If the typical dot size for a 0603 component were 0.015” to 0.018”,
the height would be approximately 0.0075” to 0.009”. If the thickness
of the stencil utilized to print the solder paste was 0.006” to 0.007”
this might not allow the glue dot to contact the component body on some
types of board finishes. For example, a typical HASL finish is
approximately 0.003” thick. If the thickness of the stencil utilized
were 0.007”, the adhesive dot would have to be at least 0.011” to 0.012”
tall to properly contact the component. This would require
approximately a 0.022” diameter dot. This is why the rheology of the
adhesive is so important. If the adhesive slumps at all after
dispensing, it may not properly contact the component. The nozzle
design also plays a part in the development of the correct dot for each
application.

Nozzle Design

When selecting a nozzle for use in dispensing adhesives the main characteristics that must be considered are nozzle
design, standoff size and placement, and nozzle ID. A relationship
exists between these characteristics and the adhesive dot diameter.
When the adhesive volume is dispensed, the surface tension of the
adhesive on the board, should be twice that of the surface area of the
adhesive at the nozzle tip. If this condition exists, as the nozzle
retracts, the adhesive will snap off clean from the nozzle and leave a
well-defined dot of constant volume on the board. The nozzle must be
chosen based upon the size dot that is required by the application.

Nozzle selection
refers in this case to specific nozzle specifications for a known dot
size requirement. The dot size requirements can be derived from the
board design being utilized or specifically the pad spacing of
components. Reference pad spacing previously discussed in this paper. It
is not uncommon for Manufacturing Engineer personnel or Quality
Engineering personnel of a printed circuit board manufacturing facility,
to inquire what a recommended adhesive dot diameter should be for a
particular component type. Much has been written in regards to
recommended surface mount component pad designs and layouts for bottom
side applications. Topside pad designs are also used on bottom side PCB
fabrication. However these guidelines are rarely utilized. The pad
spacing for a particular component for each individual customer product
is unique.

Because the pad
spacing for most typical surface mount components is not standardized
from one customer product to another, it becomes a challenging task when
recommending what tooling should be utilized to satisfy a particular
customers’ adhesive deposition requirement for a particular component.

Note that the
volume of adhesive needed to maintain the component in place during the
high speed placement or wave solder process may be larger than possible
for some specific pad designs.

The nozzle standoff
can be defined as the distance from the tip of the dispensing surface
to the end of the mechanical standoff. The nozzle standoff is used to
maintain the distance between the PCB and the dispensing tip. Most
dispensers in use today are designed to utilize some sort of mechanical
standoff with the nozzles. The standoff usually dictates, to some
degree, the height of the dispensed dot

Typical designs for
nozzle standoffs are the castle design, the post design, or a dual post
design. For applications utilizing surface mount adhesive between pads
that have had solder paste applied to them, a single post design nozzle
is the most appropriate. In this type of application the standoff
should be set at 45 ° , 135 ° , 215 ° , or 315 ° around the pad circuitry.

When selecting the
correct nozzle ID a rule of thumb is that the nozzle ID should be one
half of the required dot diameter. This will allow for the correct dot
diameter to be dispensed so that the glue snaps away from the nozzle
without contamination. By beginning with this guideline, the
approximate nozzle diameter can be determined, and then adjusted based
upon the material utilized.

Stencil Printing Considerations

 When printing
solder paste prior to dispensing surface mount adhesive, there are some
stencil design considerations that must be taken into account. The
thickness of the stencil is important because it will determine the
height of the solder paste depositions. This also determines the
minimum height of the dot that must be dispensed in order to properly
contact and hold the component. In applications where wave soldering
will follow manual assembly, a smaller stencil thickness may be used
because the ultimate solder joint quality will be determined by the wave
soldering operation. It may also be beneficial, on pads with very
tight pads spacing, to undercut the stencil so that as much space as
possible is available for adhesive deposition.

Adhesive Curing

    When printing
solder paste and dispensing epoxy between solder pasted pads a
specialized cure cycle is required. Curing epoxy at 150º C is a
bondline temperature that should be verified with thermocouples at
various locations. Curing epoxy at temperatures above 160º C can cause
the adhesive to become brittle, leading to possible component loss
during the solder wave process. The solution for this is that the epoxy
must be cured at 150º C for about 90 seconds prior to ramping to the
reflow temp. This type of reflow takes into account the adhesive cure
as well as the solder paste reflow. Care should be taken to check the
quality of the solder joints achieved with this profile. The graph
below is a sample of what the cure cycle should look like. The final
profile should take into account the recommended profiles from both the
adhesive and from the solder paste manufacturers.

Placement Machine Considerations

 When selecting a
placement machine for use in a process utilizing the dispensing of
surface mount adhesives between solder pasted pads, it is important to
consider the accuracy and the repeatability of the placement machine
down line. In typical top side applications utilizing solder paste
printing, when the solder paste is reflowed, the forces associated with
the solder, automatically center the component, within reason, on its
pads. When glue is added to the process this does not occur because the
glue resists these forces since it is cured prior to the reflowing of
the solder paste. It is important to consider all of the machines in
the line when developing this type of process.

Typical Manufacturing Line Configurations

Traditional Bottom Side Line

  GDM Adhesive Dispenser

  Vitronics Reflow Oven

  HSP Chipshooter

Typically, a
traditional bottom side manufacturing line includes an adhesive
dispenser, a chipshooter to place the bottom side components, and an
oven to cure the adhesive. This line will be followed by a wave solder
machine, which will in turn be followed by an inspection and/or rework
station.

The first thing
that must be considered when setting up any manufacturing line is the
type of components and assemblies that are going to be used or built on
it. A traditional bottom side line can be used simply to apply glue to a
printed circuit board, place components on the board, and then cure the
glue in order to hold the parts onto the board prior and during wave
soldering and manual assembly. In this type of application the green
strength of the material determines whether components stay in place
during placement operation on the chipshooter. The post cure strength
of the adhesive determines whether or not the components will stay on
the board during manual assembly and handling. This makes the choice of
glue very important. After wave solder, using this type of line, parts
may be missing due to missing or unacceptable adhesive dots or some may
have be knocked off the board during manual assembly or handling. Care
should be taken to control the forces that these assemblies are
subjected to. This line is very basic in its functionality but can
reliably build products when implemented correctly.

Bottom Side Line With Solder Paste Application

  HSP Chipshooter

  DEK Stencil Printer

  Vitronics Reflow Oven

  GDM Adhesive Dispenser

A bottom side line,
that includes solder paste application, incorporates a system for
applying the solder paste (stencil printer or high speed dispenser), an
adhesive dispenser, a chipshooter for the bottom side components, and an
oven to cure the adhesive and reflow the solder paste. A wave solder
machine and an inspection and/or rework station will then follow this
line.

This type of
manufacturing line is more flexible than the previously discussed line.
For bottom side applications, this configuration provides greater torque
strength due to the adhesive being combined with solder paste. This
will assist in reducing the number of missing part defects present in
the assembly. This type of line also helps to reduce problems related
to the wave soldering operation (insufficient solder). In this type of
application, the dot height is important to consider because the dot
must be tall enough to contact the component even above the solder paste
deposit. Consideration also must be given to the design of the stencil
used to print the solder paste and the design of the nozzle used for
high speed dispensing operations. Both of these points can turn into
problems later if not considered properly.

Mixed Technology Top/Bottom Assembly with Solder Paste Application

      Vitronics Reflow Oven

  HSP Chipshooter

  GDM Adhesive Dispenser

  DEK Stencil Printer

  GSM Flexible Placement

A mixed technology
line for assembling top and bottom side products includes a system for
applying the solder paste (stencil printer or high speed dispenser), an
adhesive dispenser, a chipshooter to place the bottom side components, a
flexible placement machine to place top and bottom side components, and
an oven to cure the adhesive and reflow the solder paste. A wave
solder machine and an inspection station will then follow this line.
The inspection station however, should see limited use because of the
robustness of this process.

This manufacturing
line is more flexible than either of the previously discussed lines.
Like the bottom side manufacturing line with solder paste, on bottom
side applications, this configuration provides greater torque strength
due to the adhesive being combined with solder paste. This assists in
reducing the number of missing parts present in the assembly. This
also helps to reduce problems related to the wave soldering operation
(insufficient solder). In this type of application, the dot height is
important to consider because the dot must be tall enough to contact the
component over and above the solder paste deposit. Consideration also
must be given to the design of the stencil used to print the solder
paste and the design of the nozzle used for high speed dispensing
operations. This line also can be used for topside applications
including the deposition of solder paste, chip placement and flexible
placement (QFPs and BGAs for example). This type of flexible
manufacturing line has become the choice for contract electronics
manufacturers because it offers a simple, total assembly solution.

Conclusion

 The dispensing of
surface mount adhesives has been a part of electronics manufacturing
since the development of surface mount components. In an effort to make
the processes involved more robust, solder paste has been added to many
manufacturing line configurations. This configuration helps to
eliminate defects such as missing components and insufficient solder
joints following wave soldering.

 In order to
implement this process there are a lot of considerations that must be
taken into account. The type of adhesive used must have rheological
properties that allow for a tall, cylindrical dot versus the typical
Hershey kiss shaped dot. This type of dot is required to properly
adhere to the component when it is placed on top of the solder paste
deposits. In order to obtain the correct dot height, the board design
must be considered carefully. By designing in the correct pad spacing,
implementation of this process is much simpler. The volume of solder
paste required must then be determined as well as the design of the
stencil. The required adhesive dot size must be considered when
designing the stencil. After the board is designed and the volume of
solder paste required has been determined, a nozzle must be designed to
provide the correct dot diameter with standoffs that will not become
contaminated with solder paste. After the adhesive is deposited and the
chips have been placed, the glue must be cured and the solder paste
must be reflowed. The profile used for this process must be developed
from the adhesive and the solder paste manufacturers’ recommended
profiles. Finally, the type of assemblies that are going to be built
must be considered when developing a manufacturing line that will meet
your needs now and in the future.

 By carefully
considering all aspects of your manufacturing process, the dispensing of
surface mount adhesives between solder pasted pads can help eliminate
defects associated with typical electronics manufacturing processes.
This process helps to eliminate problems such as insufficient solder
joints. In applications where only glue was previously utilized, this
type of process can help eliminate defects such as missing components,
that can occur as a result of handling and manual assembly. By taking
time to consider the characteristics of your manufacturing process, the
correct line configuration and process parameters can be developed to
build the highest quality assemblies possible.