Transistor Mounting and Soldering

Application Note

AN3025

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Rev. 3

Introduction

There are three basic steps recommended to mount and solder RF power transistors into a circuit.

Solder pre-tin the transistor leads

Mount the transistor

Solder the transistor leads to the circuit trace

Solder Pre-Tinning

Pre-tinning is an effective technique to improve solderability. The purpose of pre-tinning is to remove the gold

from the lead of the transistor prior to soldering the device onto the circuit board. Thus, pre-tinning reduces de-

fects associated with cold solder joints when gold is mixed with Tin-Lead (Sn-Pb) solder. It has also been ob-

served that excessive gold in a solder joint can cause solder embrittlement. After time, depending on the environ-

mental stress applied to the solder joint, cracks and even an open circuit may result. Worse still there may be a

period of time where the joint is intermittently open or closed, thus making troubleshooting extremely difficult.

Therefore, pre-tinning and removing the gold from the leads prior to soldering is a prerequisite for reliable solder

connections.

There are several methods for performing pre-tinning. One method uses a soldering iron and solder wick. First,

apply solder to the transistor lead with the soldering iron, and then remove the solder with the wick. Care must be

taken to ensure that most of the gold is removed from the lead. Also, ensure that after wicking the excess solder,

the lead remains smooth and flat.

A better process is known as “double dip” tinning. For this process, two solder pots are used. The type of solder

used will depend on the properties of the PCB/carrier. Note that the exact same solder is used in both solder

pots. The double dip method is performed in the following manner:

Dip the leads into the solder flux being used, and then dip the leads into the molten solder contained in the first

solder pot. Remove from the first pot and dip the leads into the molten solder of the second pot.

As indicated earlier, the intent is to re-flow and remove the gold from the leads; therefore the solder in the first

pot becomes contaminated with gold. The second solder pot does not have gold in solution, and therefore de-

posits only pure solder onto the leads. This method can be carried out with a great deal of success for relatively

high volumes.

If a “hands off” approach is desired, machines can be utilized to automate the lead tinning process. The ma-

chines generally work on the same principle of removing the gold-laden solder and replacing it with gold free sol-

der, but they usually have a single solder pot working as a solder wave to achieve the gold free solder joint. Out-

side contract tinning facilities are also available.

Mounting the Transistor

Prior to mounting the transistor onto the circuit, a design review of the mounting surface and transistor is very

important. Verify that the two mating surfaces are compatible with regards to thermal expansion/contraction cycle

that the system will undergo. Mismatches may be overcome by providing some form of stress relief in the transis-

tor lead itself.

Verify that the device is clean and free of burrs. The flange flatness specification is generally .002”. Mounting

surface finish roughness (Ra) should be in the order of 0.8um or 0.03 mils, If the flange or mating surface is not

adequately flat, it may bend and transmit stress to the package and perhaps cause cracking of the die or other