Nexperia AN10343, Примечание к применению, Страница: 6 / 32

Nexperia
AN10343
MicroPak soldering information
Table 4. Typical stencil thicknesses
IC package pitch Stencil thickness
≥0.5 mm 150 µm
0.4 mm to 0.5 mm 100 µm or 125 µm
≤0.4 100 µm
3.4. MicroPak 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 it 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.
Table 5 gives typical placement tolerances as a function of the IC package terminal pitch.
Table 5. Typical placement accuracies
Package terminal pitch Placement tolerance
≥0.65 mm 100 µm
<0.65 mm 50 µm
3.5. Reflow soldering
The most important step in reflow soldering is reflow itself, when the solder paste deposits melt and
soldered joints are formed. This is achieved by passing the boards through an oven and exposing
them to a temperature profile that varies in time. A temperature profile essentially consists of three
phases:
1. Preheat: the board is warmed up to a temperature that is lower than the melting point of the
solder alloy
2. Reflow: the board is heated to a peak temperature that is well above the melting point of the
solder, but below the temperature at which the components and board’s Organic Solderability
Preservative (OSP) finish are damaged
3.
Cooling down: the board is cooled down rapidly, so that soldered joints freeze before the board
exits the oven
The peak temperature during reflow has an upper and a lower limit:
• Lower limit of peak temperature; the minimum peak temperature must be at least high enough
for the solder to make reliable solder joints; this is determined by solder paste characteristics;
contact your paste supplier for details
• The upper limit of the peak temperature must be lower than:
• the maximum temperature the component can withstand according to the specification
• the temperature at which the board or the components on the board are damaged (contact
your board supplier for details)
Examples of a general purpose Pb-free reflow profile are shown in Fig. 1 and Table 6 .
AN10343 All information provided in this document is subject to legal disclaimers.
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Nexperia B.V. 2021. All rights reserved
application note Rev. 3.0 — 31 May 2021 6 / 32