IC to IC distance for Thermal Design  - Page 1
Forum section: electronics / projects

author: rakeshm55
author_rank: Regular Contributor
timestamp: January 19, 2021, 05:14:59 pm
content: Hi,In one of my projects i have two Power IC (LTM4613) close to each other 20mm apart . The Temperature above the ICs are about 68^C . I guess thermal dissipation of the chips will improve if the distance is increased from other hot components ie in this case increase distance between these two ICs.What will be the optimal distance between the ICs beyond which there will be no marginal improvement of thermal characteristics?? If there are no thumb rules then any method to identify it.

author: jbb
author_rank: Super Contributor
timestamp: January 19, 2021, 07:42:48 pm
content: The 'thermal island' effect will depend on things like PCB layout (big copper planes spread the heat), case/airflow details and any thermal management stuff (e.g. thermal pads, heat sinks). It looks like you're already using a pretty efficient converter, which helps with heat.The first question - especially if you're far through the design process - should probably be: do you need to do better? Will you have OK operating temperatures under the expected worst case conditions?Follow ups:do both chips dissipate power at the same time? If they have differing duties then maybe their mutual heating isn't a big factor (i.e. does only one work hard at a time)could you add some thermal pad or something to help get the heat out of the product?a thermal camera can be helpful for seeing heat patternsmaybe you could measure their mutual heating effect. Could you build up a test where you only load one regulator at a time? If you could measure how much A (loaded) heats B (off), followed by how much B (loaded) heats A (off) then you could characterise how much it happens(And, if mitigation is required, characterise how much your various mitigations accomplish.)converter efficiency will likely drop a little with temperature, so it's good to do validation testing in a suitable enclosure and oven

author: ConKbot
author_rank: Super Contributor
timestamp: January 20, 2021, 08:14:42 am
content: "General wisdom" is that for lateral thermal condition on typical PCB copper thicknesses, more than about an inch/25mm is going to be sharply diminishing returns on terms of getting heat out of the chips. I.e. More than 50mm between your devices will have lesser and lesser results. Onsemi appnote AND9596/D for more information. Not saying a bit more than 50mm wouldn't provide sight improvements, just don't go expecting 200mm apart to be better in a meaningful way than 100mm. (Assuming these are sunk to a ground plane our power plane with vias where doable)Obviously that's pretty handwavy, and making sure that it's not just slowly hestsoaking the case interior, along with the other things jbb brought up can be considered for a more refined choice.At least Analog/LTs thermal numbers look to be based on it being on a board. Also note the board they mention is 76mm, so about 30mm from the edge of the converter to the board edge. I'm guessing this is no accidentI've seen some cheeky shit in datasheets. Thermal resistance on an alumina substrate (it wasn't a RFIC or something you'd really expect an alumina board to be a usual option for it either). Coilcraft has this gem.QuoteSo at least Analog is playing it truthful here.

author: rakeshm55
author_rank: Regular Contributor
timestamp: January 25, 2021, 09:34:53 am
content: Quote from: jbb on January 19, 2021, 07:42:48 pm68^C at room temperature (30^C) may be a problem I am supposed to operate the device in 60^C with natural convection. I have not tested the product in chamber. Elaborate test scheme is pending. I hada provision to connect the IC top surface to Chassis via thermal pad. I did a cursory measurement wherein temp at the other side of chassis (opposite to ICs contact surface to chassis were about 44*C)Quote from: jbb on January 19, 2021, 07:42:48 pmYes both dissipate at the same timeQuote from: jbb on January 19, 2021, 07:42:48 pmAttached image. Image is of plain PCB with no chassis or shield. As "ConKbot"suggested may be I can try increasing the distance between these two devices to 50mmAt the PCB bottom side there is exposed GND pad provided with some 10 thermal vias so that shield can come in contact with a 1mm thermal pad.

author: jbb
author_rank: Super Contributor
timestamp: January 28, 2021, 09:13:20 pm
content: Well, if you’re not too far through design/testing, separating a bit more would help. But likely only a few degrees. Also, inevitably someone will ask you to add more stuff to the board, so it’s good to have a little free space on hand.Does the hardware in this image include the thermal pad you mentioned?

author: rakeshm55
author_rank: Regular Contributor
timestamp: January 29, 2021, 06:27:17 pm
content: Quote from: jbb on January 28, 2021, 09:13:20 pmNo. Thermal pads are used to fill the air gap between component and Shield/chassis