application note AN260/0597 in plastic power packages by r. tiziani handling and mounting ics integratedcircuits mounted in plastic power packagescan be damaged,or reliability compromised, by inap- propriate handlingand mountingtechniques.avoiding these problems is simple if you follow the suggestions in this section. advances in power package design have made it possible to replace metal packages with more eco- nomical plasticpackages in many high power appli- cations. most of sgs-thomson microelectronics power integrated circuits, for example, are mounted in some innovative packages, like multiwatt, flexiwatt and clipwatt, developed originally for high poweraudio amplifiers. though the intrinsic reliability of these packages is now excellent, the use of inappropriate techniques or unsuitable tools during mechanical handlingcan affectthe long term reliability of the device, or even damage it. with a few simple precautions, careful designers and pro- duction engineers can eliminate these risks, saving both time and money. bending and cutting leads the first danger area is bending and cutting the le- ads. in theseprocesses it is important to avoid strai- ning the packageand particularlythearea wherethe leads enter the encapsulating resin. if the packa- ge/lead interface is strained the resistance to humi- dity and thermal stress are compromised, affecting reliability. there are five basic rules to bear in mind : ? clamp the leadsfirmly between the package and the bend/cut point (figure 1). ? bend the leads at least 3mm from the package (figure 2a). ? never bend the leads more than 90 and never bend more than once (figure 2b). ? never bend the leads laterally (figure 2c). ? make sure that he bending/cutting tool does not damage the leads. figure 1 : clamp the leads between the package and bend/cut point. figure 2 : bend the leads at least 3mm, never bend leads more than 90 and never splay the leads out. (a) (b) (c) 1/7
insertion when mounting the ic on a printed circuit boardthe golden rule is, again, to avoid stress. in particular : ? adhereto the specifiedpin spacing of the device; don't try to bend the leadsto fit non-standardhole spacing. ? leave a suitable space between the ic and the board. if necessary use a spacer. ? take care to avoid straining the device after sol- dering. if a heatsinkis used and it is mounted on the pc board it should be attached to the ic be- fore soldering. soldering the greater danger during solderingis overheating. if an ic is exposed to high temperaturefor an exces- sive period it may be damagedor reliability reduced. recommended soldering conditions are 260 c for ten seconds or 350 c for three seconds. figure 3 shows theexcessjunction temperatureof a penta- watt package for both methods. it is also important to use suitable fluxes for the sol- dering baths to avoid deterioration of the leads or package resin. residual flux between the leads or in contactwith the resinmust be removedto guaran- tee long term reliability. the solvent used to remove excess flux should be chosen with care. in particu- lar, trichloroethylene (chci : cci 2 ) - base solvents should be avoided because the residue could cor- rode the encapsulant resin. heatsink mounting to exploitthe full capability of a power device a sui- table heatsink must be used. the most important figure 3a: the excess junction temperature of a pentawatt package in the sug- gested soldering conditions. (350 c for three seconds) figure 4 : the thermal resistance of a multi- watt package is improved by sili- con grease. here the contact thermal resistance is plotted against insulation layer thickness. figure3b: the junction temperature of a pentawatt package in the sug- gested soldering conditions. (260 c for ten seconds aspect from the pointof view of reliability is thatthehe- atsink is dimensioned to keep the junction tempera- ture as low as possible. from a mechanical point of view, however, the heatsinkmust be designedso that it does not damage the ic. care should also be taken in attaching the ic to the heatsink. the contact thermal resistance between the device and the heatsinkcanbe improved by addinga thin la- yer of silicon grease with sufficient fluidity to ensure uniform distribution. figure 4 shows how the thermal resistance of a multiwatt packageis improved by silicone grease. an excessively thick layer or an excessively viscous silicon grease may have the opposite effect and could cause deformation of the tab. application note 2/7
sgs-thomson plastic power packages - multi- watt, heptawatt, pentawatt and to220 - are attachedto the heatsink witha single screw, fle- xiwatt is attached with two screws. a spring clip may also be used as shown in figure 5b. the screw should be properly tightened to ensure that the pac- kage makes good contactwith the heatsink.it should not be too tight or the tab may be deformed,breaking the die or separating the resin from the tab or in the case of flexiwatt packages an high tightening torque may cause cracks in the molded case. the appropriate tightening torque can be found by plottingthermal resistance againsttorque as shown in table1. suggested maximum torques for m3 screws are 8kgxcm for to220, pentawatt, heptawatt and multiwatt packages.particularly for flexi- watt package, clipping system is strongly recom- mended. if different screws, or spring clips, are used, the force exerted by the tab must be equiva- lent to the force produced with these recommended torques. if clips are used, it should be taken care of the con- tact area between the plastic case and the clip: the maximum pressureallowed on plastic is 15kg/mm 2 . over this value, cracks in the molded body may ap- pear. therefore clips have to be round or smooth in the contact area to avoid concentrate loads on pla- stic body of the package. figure 5a: multiwatt, heptawatt, pentawatt, to220 and flexi- watt packages are attached to the heatsink with one or two screws. torque r th (j-heatsink) with grease r th (j -heatsin k) without grease kg ? cm c/w c/w 3 4 5 7 10 12 15 1.0 0.94 0.89 0.84 0.83 0.82 0.82 1.7 1.63 1.59 1.55 1.52 1.51 1.51 t est device: p638 (160 x 160 sq. mils) rth ( j-c) = 0.6 c/w table 1 : table resistance junction to heatsink de- pends on tightening torque in multi- watt package even if the screw is not overtightenedthe tab can be deformed, with disastrous results if the surface of the heatsink is not sufficiently flat. the planarity of the contact surface between device and heatsink must be better than 50 m m for to220, penta- watt, heptawatt and multiwatt packages. similar problems may arise if the screwhead is too narrowcompared totheholein theheatsink(figure 6). the solution here is to use a washerto distributethe force over a wider area. an alternative is to use screws of the type shown in figure 7 which have a figure 5b: multiwatt, heptawatt, pentawatt, to220 and flexi- watt packages are attached to the heatsink with a spring clip. application note 3/7
figure 7 : the recommended screw type looks like this. wide flat head. when self-tapping screws are used it is also important to provide an outlet for the mate- rial deformed as the thread is formed. poor contact and broken die will result if this is not done. another possiblehazardarises when the hole in the heatsink is formed with a punch: a circular depression may be formed around the hole, leading to deformation of the tab. this may be cured by using a washer or by modifying the punch. serious reliability problems can be encountered if the heatsink and printed circuit board are not rigidly connected. either the heatsink must be rigidly atta- ched to the printed circuit board or both must be se- curely attached to the chassis. if this is not done the stresses and strains inducedby vibration will be ap- plied to the device and in particular to the lead/resin interface. this problem is more likely to arise when large boards and large heatsinks are used or when- ever the equipment is subjectedto heavy vibrations. figure 6 : the heatsink tab may be deformed it a washer or a wide-headed screw is not used. flexiwatt#2 - heatsink mounting figure 8. * max pressure on plastic = 150 n/mm 2 . application note 4/7
flexiwatt#2 - heatsink mounting (continued) figure 9. figure 10. * max pressure on plastic = 150 n/mm 2 . * max pressure on plastic = 150 n/mm 2 . application note 5/7
flexiwatt#2 - heatsink mounting (continued) figure 11. figure 12. * max pressure on plastic = 150 n/mm 2 . -usen 2 screws m3 with interposed washers (max pressure on plastic = 150 n/mm 2 ). - max torque = 50 n*cm. - max external heat sink unflatness = 30 m m. application note 6/7
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no respon- sibility for the consequences of use of such information nor for any infringement of patents or other rights of third parti es which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specification mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. sgs-thomson microelectronics products are not authorized for use as criti- cal components in life support devices or systems without express written approval of sgs-thomson microelectronics. ? 1997 sgs-thomson microelectronics printed in italy all rights reserved ? multiwatt, pentawatt, are registered trademarks of companies belonging to the sgs-thomson microelectronics group. ? clipwatt,flexiwatt, heptawatt,powerso-20, are trademarks of companies belonging to the sgs-thomsonmicroelectronics group. sgs-thomson microelectronics group of companies australia - brazil - canada - china - france- germany - hong kong - italy - japan - korea - malaysia - malta - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. application note 7/7
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