transistor mounting and soldering application note AN3025 ? north america tel: 800.366.2266 / fax: 978.366.2266 ? europe tel: 44.1908.574.200 / fax: 44.1908.574.300 ? asia/pacific tel: 81.44.844.8296 / fax: 81.44.844.8298 1 visit www.macom.com for additional data sheets and product information. m/a-com technology solutions inc. and its affiliates reserve the right to make changes to the product(s) or inform ation contained herein without notice. rev. 3 introduction there are three basic steps recommended to mount a nd 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 solderabilit y. the purpose of pre-tinni ng 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 caus e 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 cl osed, 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 t he 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 di p 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 removi ng 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 compatib le 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 fl ange 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 packa ge and perhaps cause cracking of the die or other
transistor mounting and soldering application note AN3025 ? north america tel: 800.366.2266 / fax: 978.366.2266 ? europe tel: 44.1908.574.200 / fax: 44.1908.574.300 ? asia/pacific tel: 81.44.844.8296 / fax: 81.44.844.8298 2 visit www.macom.com for additional data sheets and product information. m/a-com technology solutions inc. and its affiliates reserve the right to make changes to the product(s) or inform ation contained herein without notice. rev. 3 elements of the transistor. it is also important to ensure that any machined features, su ch as holes, are checked for burrs around the edges. burrs have been known to cause bending stress and subsequent cracking. the bottom of the device should be coated with heatsink compound prior to mounting. m/a-com recommends a heatsink compound similar to gc electronics part number 108109. the layer of heatsink compound applied should be thin since it is only needed to fill the micr o-gaps between mating surfaces. if too much compound is applied, it may act as a thermal insulator. torque the device should be placed firmly in the channel. lead l ength should be kept as short as possible to minimize stray inductance. in order to provide adequate clea rance between the package body and mounting hardware, size 4-40 screws with split-lock and flat washers should be used. torque is to be applied in 2 steps: first, finger- tighten to 0.5 kg-cm or 0.4 inch-pounds on each side. se cond, apply controlled torque with a controlled torque screwdriver to 6 kg-cm +/- 1kg-cm or 5.2 inch-pounds +/- 0.8 inch-pounds on each side. excessive torque may damage the device. the use of washers is recommended to control the torque. the leads may now be soldered to the pcb. soldering the leads when performing the final soldering of the leads to the pcb, accepted soldering practices should be followed. it is important to verify the compatibility of the solder types. it is also important to use the appropriate flux for the desired application (i.e. rosin base d, no-clean, lead free, etc.). the temperature of the soldering iron must be controlled so that adequate heat is applied to re-flow the solder. if the temperature is too low, a cold solder joint or other nonconformity may result. afte r soldering, remove excess flux with a suitable cleaning solution. the m/a-com rf power hybrid operations soldering procedure m/a-com uses the following methods for soldering a transistor onto a pc board: the pcb is first pre-tinned so that there is solder below the lead. we t hen manually solder the leads onto the board using solder type sn62 or sn63 with rma flux 197. (the melting points of sn62 and sn63 solders are 179 ? c and 183 ? c respectively). while soldering the leads of the transistor onto the board, we hold them in place with a probe. the temperature of the solderin g iron should be maintained at approximately 250 ? c. if appropriate, the pcb can be placed on a hot plate set to 200 ? c prior to soldering the leads with the iron. this improves the flow of the solder, thus impr oving the reliability of the solder joints. we use a metcal soldering system mx-500 with a #137 tip. the tip size should be selected according to the package lead width. metcal has a wide range of models with temperature sensing capability at the tip. other companies such as weller and edsyn make solder systems that should also work well. summary these suggestions should only be used as general instructions for mounting tr ansistors into the next assembly. care should be taken if the transistor is immersed in any fl ux, solvent, or cleaning solution. however, this is gen- erally not a concern for hermetically sealed devices that do not use an epoxy material for lid sealing. an ultra- sonic cleaning system should not be used as the vibrat ion could damage the transistor. lastly, good esd prac- tices should be employed throu ghout the assembly operation.
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