choosing the right sil-pad star ts with the mechanical and electrical pr oper ties m echanical p roper ties w oven ber glass and lms ar e used in sil-pads to pr ovide mechanical r einfor cement. the most impor tant mechanical pr oper ty in sil-pad applications is r esistance to cut-thr ough to avoid electrical shor ting fr om the device to the heat sink. ?spk4 , spk6 ? and spk10 ar e ver y goo d at r esisting cut-thr ough fr om sharp bur rs left on heat sinks after machining operations such as drilling and tapping ?fiber glass is goo d at r esisting the type of cut-thr ough encounter ed when device mounting anges ar e pulled into oversized mounting holes. this occurs when fasteners ar e tor qued. (sp400 , sp1000 , sp2000 ) cut-thr ough r esistance is ver y dependent on the application and depends on several factors: ?a ver y sharp bur r may cause cut-thr ough with less than 100 pounds while a blunt bur r may r equir e several hundr ed pounds to cause cut-thr ough ?when two at parallel sur faces ar e br ought together on a sil-pad, over 1000 pounds of for ce can be applied without damaging the insulator ?the poly-pad insulators ar e the most mechanically durable sil-pads overall. the polyester r esin used has a higher mo dulus than silicone r ubber . (poly-pad 400 , poly-pad 1000 , poly-pad k4 and poly-pad k10 ) m ounting t echniques and m ounting p ressure t ypical mounting techniques include: ?a spring clip, which exer ts a centralized clamping for ce on the bo dy of the transistor . the gr eater the mounting for ce of the spring, the lower the ther mal r esistance of the insulator ?a scr ew in the mounting tab. w ith a scr ew mounted to-220, the for ce on the transistor is deter mined by the tor que applied to the fastener in extr emely low pr essur e applications, an insulator with pr essur e sensitive adhesive on each side may give the lowest ther mal r esistance since the adhesive wets out the inter face easier than the dr y r ubber . this decr eases the inter facial ther mal r esistance. devices with lar ger sur face ar eas need mor e pr essur e to get the insulator to confor m to the inter face than smaller devices. in most scr ew mount applications, the tor que r equir ed to tighten the fastener is suf cient to generate the pr essur e needed for optimum ther mal r esistance. ther e ar e exceptions wher e the speci ed tor que on the fastener does not yield the optimum ther mal r esistance for the insulator being used and either a dif fer ent insulator or a dif fer ent mounting scheme should be used. inter facial ther mal r esistance decr eases as time under pr essur e incr eases. in applications wher e high clamping for ces cannot be used, time can be substituted for pr essur e to achieve lower ther mal r esistance. the only way to know pr ecisely what the ther mal r esistance of an insulator will be in an application is to measur e it in that application. 2
e lectrical p roper ties if your application does not r equir e electrical insulation q-pad ii or q-pad 3 ar e ideal gr ease r eplacement materials. these materials do not isolate but have excellent ther mal pr oper ties. the most impor tant electrical pr oper ty in a typical assembly wher e a sil-pad insulator is used is dielectric str ength. in many cases the dielectric str ength of a sil-pad will be the deter mining factor in the design of the apparatus in which it is to be used. her e ar e some general guidelines r egar ding electrical pr oper ties to consider when selecting a sil-pad material; ?q-pad ii and q-pad 3 ar e used when electrical isolation is not r equir ed ?dielectric br eakdown voltage is the total voltage that a dielectric material can withstand. when insulating electrical components fr om each other and gr ound, it is desirable to use an insulator with a high br eakdown voltage ?br eakdown voltage decr eases as the ar ea of the electr o des incr eases. this ar ea ef fect is mor e pr onounced as the thickness of the insulator decr eases ?br eakdown voltage decr eases as temperatur e incr eases br eakdown voltage decr eases as humidity incr eases (sp1750 and sp1950 ar e less sensitive to moistur e) br eakdown voltage decr eases in the pr esence of par tial dischar ge ?br eakdown voltage decr eases as the size of the voltage sour ce (kv a rating) incr eases ?br eakdown voltage can be decr eased by excessive mechanical str ess on the insulator dielectric str ength, dielectric constant and volume r esistivity should all be taken into consideration when selecting a sil-pad material. if your application r equir es special electrical per for mance please contact the factor y for mor e detailed testing infor mation. breakdown dielectric dielectric volume voltage strength constant resistivity material (kv) (volts/mil) ( kv/mm) (ohm-metre) sp400 -.007 5 700 18 5.5 10 11 sp400 -.009 7 800 20 5.5 10 11 sp1000 7 700 18 4.5 10 11 sp2000 12 800 20 4.0 10 11 spk-4 7 1200 30 5.0 10 12 spk-6 7 1200 30 4.0 10 12 spk-10 7 1200 30 3.7 10 12 test method astm d 149* astm d 149* astm d 150 astm d 257 *method a, type 3 electrodes t ypical e lectrical p roper ties of s il -p ads 3 choosing the right sil-pad star ts with the mechanical and electrical pr oper ties
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