surface-mount devices (SMD's) onto a
printed circuit board (PCB)
consists of the following steps: 1) application of
specific locations on the board; 2) positioning of the SMD's on the
solder paste deposits which will hold them in place on the board; and 3)
solder reflow, which is a high-temperature process that melts the solder
paste so that it can form the final solder connection between the SMD's
and the board.
is accomplished using an equipment known as the solder reflow oven.
Reflow ovens employ two major techniques to expose the board assembly to
the necessary temperature profile, namely, Infrared (IR) Reflow and
Convection Reflow, which are not necessarily exclusive of each other. In
fact, many modern ovens utilize both techniques to achieve excellent
IR reflow involves the
transfer of thermal energy from infrared lamps to the board assembly.
The board assembly is heated by IR reflow primarily by line-of-sight
surface heat absorption and because of this, variations in the density
of the board can result in 'hot spots' (or localized areas with
significantly higher temperatures) on the board during IR reflow. As
such, some components experience higher stress levels than others on the
board even if they're subjected to the same IR reflow conditions.
transfers heat to the board assembly by blowing heated air around it.
Convection reflow provides a more uniform heat distribution to the
circuit assembly compared to IR reflow.
actually a third method for solder reflow, although it is used to a much
lesser extent today than the first two techniques mentioned.
as it is called, transfers heat to the board assembly by boiling inert
fluorocarbon liquid and enveloping the board with its resulting vapors.
Its major drawback is that its reflow temperature depends on the boiling
temperature of the liquid used.
reflow process follows an optimized temperature profile to prevent the
board from experiencing unrealistically high thermal stresses while it
is undergoing reflow. A
would consist of the following steps:
which consists of gradually ramping up the temperature to the preheat
zone temperature at which the solvents will be evaporated from the
which consists of bringing the dehydrated solder paste to a temperature
at which it is chemically activated, allowing it to react with and
remove surface oxides and contaminants;
which consists of ramping up the temperature to the point at which the
solder alloy content of the solder paste melts, causing the solder to
sufficiently wet the interconnection surfaces of both the SMD's and the
board and form the required solder fillet between the two; the peak
reflow temperature should be significantly higher than the solder
alloy's melting point to ensure good wetting, but not so high that
damage to the components is caused;
which consists of ramping down the temperature at optimum speed (fast
enough to form small grains that lead to higher fatigue resistance, but
slow enough to prevent thermo-mechanical damage to the components) until
the solder becomes solid again, forming good metallurgical bonds between
the components and the board.
temperatures required by Pb-free board assemblies are higher than those
required by non-Pb-free boards, mainly because
Pb-free solders generally have higher
melting temperatures than Pb-Sn solders. As such, the optimization
of the reflow profile is more critical in Pb-free assemblies with regard
to preventing the occurrence of package cracking in the surface mount
components on the board.
PCB Solder Printing;
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