ISL8200MM

FN7690 Rev 2.00

Page 20 of 24

Mar 15, 2013

Stencil Pattern Design

Reflowed solder joints on the perimeter I/O lands should have about a

50μm to 75μm (2mil to 3mil) standoff height. The solder paste stencil

design is the first step in developing optimized, reliable solder joins.

Stencil aperture size to land size ratio should typically be 1:1. The

aperture width may be reduced slightly to help prevent solder bridging

between adjacent I/O lands. To reduce solder paste volume on the

larger thermal lands, it is recommended that an array of smaller

apertures be used instead of one large aperture. It is recommended

that the stencil printing area cover 50% to 80% of the PCB layout

pattern. A typical solder stencil pattern is shown in the Package

Outline Drawing L23.15x15 on page 23. The gap width between pad

to pad is 0.6mm. The user should consider the symmetry of the whole

stencil pattern when designing its pads. A laser cut, stainless steel

stencil with electropolished trapezoidal walls is recommended.

Electropolishing “smooths” the aperture walls resulting in reduced

surface friction and better paste release which reduces voids. Using a

trapezoidal section aperture (TSA) also promotes paste release and

forms a “brick like” paste deposit that assists in firm component

placement. A 0.1mm to 0.15mm stencil thickness is recommended

for this large pitch (1.3mm) QFN.

Reflow Parameters

Due to the low mount height of the QFN, “No Clean” Type 3 solder paste

per ANSI/J-STD-005 is recommended. Nitrogen purge is also

recommended during reflow. A system board reflow profile depends on

the thermal mass of the entire populated board, so it is not practical to

define a specific soldering profile just for the QFN. The profile given in

Figure 33 is provided as a guideline, to be customized for varying

manufacturing practices and applications.

FIGURE 33. TYPICAL REFLOW PROFILE

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300

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350

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50

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DURATION (s)

SLOW RAMP (3°C/s MAX)

AND SOAK FROM +100°C

TO +150°C FOR 60s~120s

RAMP RATE £1.5°C FROM +70°C TO +90°C

PEAK TEMPERATURE +215°C~+220°C;

TYPICALLY 60s-70s ABOVE +183°C

KEEP LESS THAN 20s WITHIN 5°C OF PEAK TEMP.