AN10365_3
© NXP B.V. 2008. All rights reserved.
Application note
Rev. 03 — 22 April 2008
15 of 24
NXP Semiconductors
AN10365
Surface mount reflow soldering description
Keep in mind that printing a smaller volume of solder paste could have adverse effects on
the solder joint reliability. Also, if there are vias in pads, solder paste deposits should be
arranged so that paste is never printed directly over a via.
4.2 IC package placement
The required placement accuracy of a package depends on a variety of factors, such as
package size and the terminal pitch, but also the package type itself. During reflow, when
the solder is molten, a package that has not been placed perfectly may center itself on the
pads: this is referred to as self-alignment. Therefore, the required placement accuracy of a
package may be less tight if this package is a trusted self-aligner. It is known, for example,
that BGAs are good at self-alignment, as the package body essentially rests on a number
of droplets of molten solder, resulting in minimal friction.
Typical placement tolerances, as a function of the IC package terminal pitch, are given in
.
IC packages are usually placed with two types of machines. If the highest placement
accuracy is required, the slower but more accurate machines must be used. These
machines are also often more flexible when it comes to unusual package shapes, that
may require dedicated nozzles and non-standard trays. If the highest placement accuracy
is not necessary, and there are no special requirements, fast component mounters, or
chip shooters, can be used. These machines can process up to 100,000 components per
hour.
The placement force may also be an important parameter for some packages. In theory,
an IC package is always pressed down into the solder paste until it rests on a single layer
of solder paste powder particles - the rest of the solder paste is pressed aside. A
consequence that is immediately apparent, is that the solder paste that is pushed aside,
or that bulges outside the package, may cause bridges with neighboring solder paste
deposits.
In extreme cases, solder paste may not only bulge outside the pads, but it may actually be
blasted further away from the pads, so that a small amount of solder paste is no longer
connected to the paste deposit it originally came from. This must always be avoided, as
the splattered solder paste may cause shorts elsewhere on the board, and the solder
paste deposit it originally came from may end up with insufficient solder. Incidentally, this
effect is often caused in part by use of an improper nozzle shape, so that the paste is
actually blown away by air from the nozzle.
If the placement force is too low, there is a chance that an IC package terminal does not
make sufficient contact with the solder paste. In that case, there is a risk that the solder
paste tackiness will not be able to hold it in place up to the reflow zone in the oven, and
the package may be displaced. In addition, even if the IC package remains in place, there
may be bad contact between the package terminals and the solder paste, resulting in
open contacts or bad joints.
Table 4.
Typical placement accuracies
Package terminal pitch
Placement tolerance
≥
0.65 mm
100
µ
m
< 0.65 mm, > 0.5 mm
50
µ
m
< 0.5 mm
30
µ
m
