01005 assembly process—from the board design to reflow
Figure 3. Layout without solder mask between pads. Figure 4. 01005 placed in paste, correctly aligned. Figure 5. Component ‘pile’ after reflow.
The solder mask, especially for such small opposite side than the component on the
components, has an important function if track connecting into pad two will end up
it is well designed. It has to reliably cover tilted after reflow.
wetable areas like tracks, because uncov- The above examples prove that a good
ered tracks can cause shifted components PCB design is essential for quality, espe-
during reflow. cially for miniaturized components design
Figure 3 shows an example of tracks for manufacturability.
between pads that are not separated by
solder mask. Another design error is that solder paste application
the track width is only slightly reduced The principle of solder paste printing via
compared with the pad width. This allows stencil is that the paste that is first filled in
the solder to flow between pads without the opening (by the squeegee) has, in the
!
any barrier during reflow. When the solder moment of depositing, less adhesion to
is in the liquid state, the surface tension the side walls of the opening than to the
!
of the molten solder results in a force surface of the pad. If the adhesion to the
pulling components out of their initial side walls exceeds the one to the pad, no
position and in case of such a layout it will print is possible. Since the adhesion to the
!
!
cause misalignment. The problem is that side walls is a function of their roughness,
if the solderable pad area is not defined or it is important to select the best combi-
limited by solder mask, the movement of nation of material and manufacturing
such light weight components is uncon- methods for the stencil. Also important is
trolled. The most desirable effect is if the the area ratio that can be calculated when
!
solderable pad area is well defined by the the dimensions of the opening and the
pad design. This leads to the so-called ‘self- stencil thickness are known. Since most
Figure 6. Impact of large spacings in the solder
alignment’ of the components, which can of the time the stencil thickness and the
mask..
compensate for minor shifts in the paste opening size are not really negotiable, the
print and/or component placement. only remaining influencing factors are the
In Figure 4 you can see the accurate selection of the stencil material and the portant to have a well supported PCB. If
placement of three 01005 components method of manufacturing. Therefore, it is the PCB is very thin or below 0.5 mm, a
with 120 µm gap. However, the solder imperative that you choose a solder paste vacuum tooling will be important to flat-
paste is too much due to a relatively thick with good printing properties and a fine ten the PCB while supporting it from the
stencil with 80 µm, and consequently the enough powder size. bottom side. It is also important that the
solder paste depot on the pad is smashed The best solder paste release properties stencil is wiped off often, so that it is kept
a little. were achieved with electroformed nickel clean underneath.
In Figure 5 you can see what has hap- stencils; however, the best overall print
pened during reflow. When the solder results were achieved with laser cut nickel component placement
liquefied the components were pulled stencils. This was due to the fact that for For the placement process, the first and
together out of their predetermined posi- both stencil types the nickel is electrochem- most important step is to pick up the
tion. The result is a pile of components ically deposited, but for the electroformed component out of the feeder (Figure 7). Pre-
that, electrically, would still work fine, but stencil the openings are created with a film- condition for good component pick up is:
would not be acceptable for any quality based process, which is less accurate than
inspection. cutting the openings with a laser.
• Good component quality (all
Figure 6 shows the effect when the The solder paste powder type can be
dimensions/tolerances within
solder mask opening is too large and selected with the common rule of thumb
specification)
portions of the tracks are uncovered. You in mind that five balls of powder should fit
• Good tape quality (all dimen-
can also see that even single components through the smallest stencil opening side
sions/tolerances within specifi-
will be pulled to the side where a track is by side. The conclusion will be to use at
cation)
connected into the pad. If pad one of a least a Type 4—or better, a Type 5—paste.
• Feeder unit with sufficient posi-
component has a track connecting to the For good print results it is also im-
tioning accuracy and repeatabil
www.globalsmt.net Global SMT & Packaging – September 2008 – 13
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