<div class='quotetop'>Цитата(quazzle team @ 14.07.2009 15:14) <{POST_SNAPBACK}></div>
есть достаточно простой и технологичный способ изготовить собственный "стар" или даже "драйвер на старе" (как это сделал я). Вопрос - только добыть материал - PCB на алюминиевой подложке. Изолятор - полиамидная смола, около 5мкм. Высокочастотную часть расчитывать с учетом емкостных связей через подложку (это пожалуй единственный минус этого материала). Технология изготовления платы из него мало чем отличается от обычного FR4. Т.е. с помощью принтера или вообще фломастера и хлороного железа можно получить весьма эффективный стар. Синк нужно паять прямо на фольгу, тепловое сопротивление в таком случае будет на порядок ниже любой термопасты. И гарантируется изоляция от основы (например в тех же SSC P4 синк не изолирован от кристалла). На высоких частотах материальчик готов преподнести сюрпризы, к этому нужно быть готовым.[/b]
Привожу письмо с mtbr.com, человек спрашивал у Cree что они думают по поводу старов и Arctic Silver Epoxy.
Код:
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As promised, here is reproduced my email to Cree, and their reply :-
Hello,
We have some engineering questions re optimum thermal solutions for mounting Cree MC-E multichip leds.
Firstly, the Cree literature states that the MCE has an "electrically isolated thermal path", which presumably means that the solder point is electrically isolated from anode and cathode. Is this correct?
Answer: It is correct.
Assuming this is true, it seems strange that the commercially available MCPCBs for the MC-E electrically isolate the copper solder point on the MCPCB from the aluminium substrate of the MCPCB. As a result of this redundant electrical isolation on the MCPCB, the thermal resistance across the MCPCB appears to be typically 0.8 to 1.0 K/W. As a result, at the maximum MC-E dissipation of around 10W, the temperature rise across the MCPCB will be around 8 to 10 DegC. While this is satisfactory in most applications, it is clearly not optimum. Are we correct in saying that a superior thermal solution would be to use a copper substrate on the MCPCB, and solder the MC-E directly to this copper substrate? This should result in the semiconductor junction temperature of nearly 8 DegC lower than it would be with existing electrically isolated MCPCBs.
Answer: You are right. The commercial available MCPCB is designed for all kind of LEDs (include no electrically isolated LED). With MCE, you can do the way you described to take the advantage of electrically isolated thermal pad.
To avoid the cost and thermal penalty of an MCPCB! , in our application we are considering the use of a thermally conductive epoxy to bond the MC-E emitter directly to an aluminium heatsink. The thermally conductivity of the adhesive is 7.5 W/mK, and the area of the MC-E solder point is 14 mm^2, with the result that the thermal resistance of the epoxy interface will be around 0.95 K/W for a bond thickness of 0.1mm, which is comparable with an MCPCB. However, bond thicknesses of down to 0.025mm are obtainable giving lower thermal resistance than an MCPCB, at lower cost.
Is epoxy bonding of the MC-E emitter, as described, a practical solution? In particular, are there likely to be issues of bond failure over many thermal cycles, given the difference in expansion coefficient between the aluminium heatsink and the MC-E solder point? If so, is it satisfactory to epoxy bond the MC-E directly to copper, rather than aluminium?
Answer: This is very good question. We never tried the way you described. It will depend on the operation conditions of the LEDs. In normal condition, it should be fine.
If I may be so bold as to summarise this thread, commercially available MCPCB stars for the MC-E are poorly designed and have unecessarily poor thermal performance, and a better thermal solution is to epoxy the MC-E directly to the aluminium heatsink/casing, using Arctic Silver epoxy, taking care to ensure that the bond thickness is as thin as possible.
I find it interesting that apparently Cree has never tried the "direct epoxy" mounting method, so arguably the DIY people here are leading the way.
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Добавлено позже (14/07/2009 15:30):
<div class='quotetop'>Цитата(quazzle team @ 14.07.2009 15:14) <{POST_SNAPBACK}></div>
И гарантируется изоляция от основы (например в тех же SSC P4 синк не изолирован от кристалла)[/b]
Arctic Silver & Arctic Aluminia epoxy -- обе выступают диэлектриками. Правда, Arctic Silver может немного повышать ёмкость, поэтому тоже не везде рекомендуется...
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