AN3232

Electrical connection

Doc ID 17594 Rev 3

23/26

11

Electrical connection

Leads and/or flanges should be attached to the PCB and/or copper heatsinks using typical

Sn63Pb37 or Pb-free solders, in accordance with the supplier's recommendations.

However, the following guidelines with respect to the package should be considered:

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Avoid, as much as possible, the use of flux or flux solutions as it is a potential source of

contamination to the device.

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The leads of the transistor may be tinned prior to assembly on the PCB. This practice is

used in situations when it is undesirable to introduce even a small amount of gold into

the solder composition, forestalling the possibility of solder embrittlement issues. The

source of Au stems from the lead and/or backside surfaces, which are plated with 40 to

100 µin (1 to 2.5 µm) of Au.

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The amount of solder to be used depends on the type of solder and the amount of

allowable Au in the final solder composition. It is desirable to increase the amount of

solder when Au content is a concern, such as in Pb-free soldering applications.

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It is recommended to use either a solder preform or solder paste having a thickness of

0.001 to 0.010 in (0.03 to 0.3 mm), typically 0.006 in (.15 mm).

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Adjustment of the solder preform or paste thickness may be used advantageously to

account for thickness variations in the PCB manufacturing process.

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In cases where the backside of the flange is soldered to a copper core or heatsink, the

package design and previously discussed mechanical mounting procedure provide

sufficient downward pressure on the solder preform, resulting in a thin, high-quality

bondline after a typical solder reflow procedure. In such cases, solder-wells or a solder-

moat around the perimeter of the flange should be considered, to provide a region for

excess solder to flow.

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A solder preform or other metallic foil may be used as both a TIM and electrical

interface between the backside of the package (source) and amplifier ground (normally

the core layer or the heatsink) even without soldering.

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Electrically insulative thermal pastes must be applied sufficiently thin, such that the

downward pressure is high enough to force metal-to-metal contact over a significant

area of the surfaces between the package (source) and amplifier ground.