DocID15852 Rev 3

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TN0107

Reflow

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Reflow

The SLLIMM is intended to be a through hole device (THD). THDs are typically soldered by

wave soldering. However, as the number of THDs on a board continues decreasing in some

applications, wave soldering becomes less cost-effective, so the few remaining THDs

(mostly connectors and special components) are soldered with selective wave soldering or

with pin-in-paste techniques plus reflow soldering.

3.1

Selective wave soldering of SLLIMM

Wave soldering is a large-scale soldering process by which electronic components are

soldered to a PCB to form an electronic assembly. The name derives from the fact that the

process uses a tank to hold a quantity of molten solder; components are inserted into or

placed on the PCB and the loaded PCB is passed across a pumped wave or cascade of

solder. The solder wets the exposed metallic areas of the board (those not protected with

solder mask), creating a reliable mechanical and electrical connection.

For THDs, only the leads that extend through the drill holes in the PCB contact the hot

solder. The body of the package is heated by the hot leads. This has two consequences:

1.

The package body is cooler than in the case of reflow soldering.

2.

The temperature gradient between leads and body and inside the package is greater

than in the case of reflow soldering.

Therefore, for wave-solderable THDs, the heat resistance is tested according to JESD22-

B106 and IEC668 2-20 (typically 260 °C, 10 s).

Note:

Immersion of the whole package body into the molten solder is not recommended since the

SLLIMM is not designated for such a harsh temperature shock.

For evaluation of its ability to withstand full body immersion please see JESD22-A111.

There are many types of wave-soldering machines, but their basic components and

principles are the same. A standard wave-soldering machine consists of three zones: the

fluxing zone, the preheating zone, and the soldering zone. A fourth zone, the cleaning zone,

may be used depending on the type of flux applied.

Dual-wave soldering is the most commonly used wave-soldering method (please see

Figure 6). The peak temperatures, ramp rates, and times that are used depend on the

materials and the wave-soldering equipment.

The first wave has a turbulent flow and therefore guarantees a wetting of nearly all shapes

of leads and board pads, but also creates an increased number of unwanted solder bridges.

These solder bridges have to be removed by the second, laminar wave.

When using lead-free solder alloys, a nitrogen atmosphere is recommended.

Selective wave soldering is used when only a few THDs have to be soldered onto the board.

Generally this is done after the other components are already soldered by reflow soldering.

This requires effective protection of these components undergoing the selective wave

soldering. This protection can be achieved either by using special fixtures and deflectors for

the PCB or a small wave shape achieved by using special wave-guiding tubes or covers.