AMIS−30663
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10
Soldering
Introduction to Soldering Surface Mount Packages
This text gives a very brief insight to a complex
technology. A more in−depth account of soldering ICs can
be found in the ON Semiconductor “Data Handbook IC26;
Integrated Circuit Packages” (document order number 9398
652 90011). There is no soldering method that is ideal for all
surface mount IC packages. Wave soldering is not always
suitable for surface mount ICs, or for printed−circuit boards
(PCB) with high population densities. In these situations
re−flow soldering is often used.
Re−flow Soldering
Reflow soldering requires solder paste (a suspension of
fine solder particles, flux and binding agent) to be applied to
the PCB by screen printing, stencilling or pressure−syringe
dispensing before package placement. Several methods
exist for re−flowing; for example, infrared/convection
heating in a conveyor type oven.
Throughput times (preheating, soldering and cooling)
vary between 100 and 200 seconds depending on heating
method. Typical re−flow peak temperatures range from 215
to 250°C. The top−surface temperature of the packages
should preferably be kept below 230°C.
Wave Soldering
Conventional single wave soldering is not recommended
for surface mount devices (SMDs) or PCBs with a high
component density, as solder bridging and non−wetting can
present major problems. To overcome these problems the
double−wave soldering method was specifically developed.
If wave soldering is used the following conditions must be
observed for optimal results:
• Use a double−wave soldering method comprising a
turbulent wave with high upward pressure followed by
a smooth laminar wave.
• For packages with leads on two sides and a pitch (e):
1. Larger than or equal to 1.27 mm, the footprint
longitudinal axis is preferred to be parallel to the
transport direction of the PCB;
2. Smaller than 1.27 mm, the footprint longitudinal
axis must be parallel to the transport direction of
the PCB. The footprint must incorporate solder
thieves at the downstream end.
• For packages with leads on four sides, the footprint
must be placed at a 45° angle to the transport direction
of the PCB. The footprint must incorporate solder
thieves downstream and at the side corners.
During placement and before soldering, the package must
be fixed with a droplet of adhesive. The adhesive can be
applied by screen printing, pin transfer or syringe
dispensing. The package can be soldered after the adhesive
is cured. Typical dwell time is four seconds at 250°C. A
mildly−activated flux will eliminate the need for removal of
corrosive residues in most applications.
Manual Soldering
Fix the component by first soldering two diagonally−
opposite end leads. Use a low voltage (24 V or less)
soldering iron applied to the flat part of the lead. Contact
time must be limited to 10 seconds at up to 300°C.
When using a dedicated tool, all other leads can be
soldered in one operation within two to five seconds
between 270 and 320°C.
Table 9. Soldering Process
Package
Soldering Method
Wave Re−flow (Note 8)
BGA, SQFP Not suitable Suitable
HLQFP, HSQFP, HSOP, HTSSOP, SMS Not suitable (Note 9) Suitable
PLCC (Note 10), SO, SOJ Suitable Suitable
LQFP, QFP, TQFP Not recommended (Notes 10 and 11) Suitable
SSOP, TSSOP, VSO Not recommended (Note 12) Suitable
8. All surface mount (SMD) packages are moisture sensitive. Depending upon the moisture content, the maximum temperature (with respect
to time) and body size of the package, there is a risk that internal or external package cracks may occur due to vaporization of the moisture
in them (the so called popcorn effect). For details, refer to the drypack information in the “Data Handbook IC26; Integrated Circuit Packages;
Section: Packing Methods.”
9. These packages are not suitable for wave soldering as a solder joint between the PCB and heatsink (at bottom version) can not be achieved,
and as solder may stick to the heatsink (on top version).
10.If wave soldering is considered, then the package must be placed at a 45° angle to the solder wave direction. The package footprint must
incorporate solder thieves downstream and at the side corners.
11. Wave soldering is only suitable for LQFP, TQFP and QFP packages with a pitch (e) equal to or larger than 0.8 mm; it is definitely not suitable
for packages with a pitch (e) equal to or smaller than 0.65 mm.
12. Wave soldering is only suitable for SSOP and TSSOP packages with a pitch (e) equal to or larger than 0.65 mm; it is definitely not suitable
for packages with a pitch (e) equal to or smaller than 0.5 mm.
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