SMART Group launches “Lead-Free Process Defect Guide 2”
Incomplete solder coverage
Lead-free wave soldering often results in
solder joints with reduced hole fill. This
can be related to the process parameters,
the solder finish on the surface of the
printed board or the flux type. Each
step needs to be examined to pinpoint
the actual cause. Due to the increased
temperatures of lead-free processes, the
design on multilayer boards can have a
great influence on the solder, causing
it to solidify prematurely due to heat
loss on the inner layer connections.
Figure 3. Three joints on a demonstration
Wherever possible the inner layer Figure 6. Excess solder paste under the centre
package, with one open connection.
ground or earth plane connection should
pad of the packaging can cause the part to float
be modified, increasing the clearance
on the surface of the solder.
Open joint on POP package between the holes and the inner layer,
‘POP package’ refers to package-on-package reducing the number of connections or LGA floating on reflow
assembly or stack where two or more ball reducing the track connection width. Open solder joints caused by lifting of a
grid array (BGA) or chip scale package The two example joints in Figure 4 show land grid array (LGA) package during
(CSP) components are placed on top of incomplete solder coverage on the top reflow is a very common problem. This is
each other and reflow soldered together. surface of the board; however both easily normally very easy to overcome with good
One of the main reasons for open solder exceed the minimum requirements of stencil design. The most common reason
connections is package-to-package warp. the IPC610 level 3 and should not for problems on these parts is excess solder
This can lead to one or multiple open be reworked. paste under the centre pad of the package
connections, which are very difficult to causing the part to float on the surface of
detect during component manufacture or the solder. As a basic guide, the centre
assembly. Figure 3 shows three joints on pad stencil aperture should be reduced by
a demonstration package with one open 50-60% of the area of the pad. Depending
connection. Normally this type of defect on the size of the pad the area should be
is easily detected during x-ray, but with split up with four, six or even nine equally
multiple balls on top of each other, it spaced apertures. This modification reduces
is challenging. float on reflow and increases the chance
of the volatile elements of solder paste
escaping, reducing void formation at the
joint interfaces.
Figure 5. Poor solderability can be caused by
a lack of protective coating.
Solderability failure on LGA
Quad flat no lead (QFN) and land grid
array (LGA) packages often do not have
solderable side terminations, or they are
poorly solderable due to the component
manufacturing process. There is not
requirement for the side terminations to be
solderable based on IPC 610 requirements;
however it is often the case that customers
Figure 7. Inadequately packaged trays of MSDs.
like to see a joint and it makes automatic
optical inspection (AOI) possible. Poor Damage of component
solderability of the edge termination is Moisture sensitive devices (MSDs) must be
normally caused by a lack of a protective correctly packaged to prevent them being
coating, like tin on the surface of the mechanically damaged during handling or
copper lead frame or resin smearing during cracking when they pass through the reflow
component singulation. Solderability is process. The waffle tray of components in
best tested using a wetting balance or in Figure 7 has a thick plastic band around
production with paste and a glass slide to the trays that would not be adequate to
Figure 4. Two examples of incomplete solder
simulate reflow. maintain the trays’ position if dropped. In
coverage related to lead-free wave soldering.
the case of fine pitch gull wing parts, this
6 – Global SMT & Packaging - August 2008 www.globalsmt.net
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