Soldering SSOP exposed pad for thermal dissipation - Page 1
Forum section: electronics / projects

author: MattHollands
author_rank: Frequent Contributor
timestamp: August 03, 2015, 03:21:53 pm
content: Hi! I am developing a project which uses an SSOP28 package with exposed pad (for thermal dissipation, no electrical connection required). For development purposes I would rather have it as a DIL package so I've bought some breakout boards from ebay. However they do not feature solder pads for the exposed pad.In order to dissipate heat, would it be reasonable to solder the chip down with some aluminium foil underneath (in contact with the exposed pad), which would protrude from the top of the chip? This could carry away some heat from under the chip. I would of course be careful to avoid shorts and would put plastic on the underside of the foil to stop it shorting any of the pins.Is there any chance of this working? Or does anyone else have any other suggestions? Worst comes to worst I could live without the exposed pad soldered and be careful with the temperature.

author: georges80
author_rank: Frequent Contributor
timestamp: August 03, 2015, 03:33:07 pm
content: Your aluminium foil idea doesn't seem feasible to me. How is the foil going to attach to the thermal pad area? It's also so thin as to be pretty useless.I use a lot of thermal pad devices (switcher IC's, FETs etc) and just get prototype PCB's done. For ~$70 delivered and typically within 1 week I see no reason to kludge a design up to 'test' things. Especially with many designs the PCB layout is critical so putting things on breadboards with adapters would fail to even work.Thermal pads need to be soldered to the PCB and also have multiple vias to carry the heat through to internal planes or to a rear copper area to connection to a heatsink via thermal tape etc.Most devices that I've used that require thermal dissipation via the pad DO need an electrical connection since it is often the power ground (for switcher IC's) or a power path (FETs etc). Only on microcontrollers is the pad just 'another' ground and it is not for thermal requirements but to ensure the package is reliably soldered/held down to the PCB (e.g. some of the Atmel QFN packages).Anyhow without knowing anything about what you thermal requirements are (the actual IC part number, its thermal resistance, the power levels you plan to run etc etc) how can anyone tell you what may or may not work??cheers,george.

author: tszaboo
author_rank: Super Contributor
timestamp: August 03, 2015, 03:49:33 pm
content: What I did was, I took a chunk of copper bar, around 5mmx5mmx 10cm, didnt really measure it. Mill out the middle of the adapter. Solder the bar to the chip. Solder the rest of the pins to the adapter. put the bar in water, or add additional heatsinking if required, It wasn't for me.You wont be able to solder aluminium because the oxid layer on it. Go for copper.

author: MattHollands
author_rank: Frequent Contributor
timestamp: August 03, 2015, 03:51:38 pm
content: Thanks for your message, GeorgeIf I folded the foil then perhaps it would be thick enough to be held in contact by pressure? And if enough is protruding from the board then it would effectively be like having a pad on the board to dissipate heat, except that it is not glued down to the board?I definitely cannot afford the money, as a student, to be buying boards without some testing before hand, especially as I am just starting development.This is the chip I am usinghttp://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00096923.pdfIt features a power op-amp to drive the mains circuit so I imagine that is what will be generating most of the heat.For now I would just be messing around with communicating with the chip over SPI, not actually using the power op-amp at all, so I reckon that would be fine in terms of heat dissipation until I start actually driving any load. I think the op amp will be driving about 0.5A at 6V, so 3W.The datasheet makes no reference to the exposed pad other than in discussion of thermal properties.

author: MattHollands
author_rank: Frequent Contributor
timestamp: August 03, 2015, 03:52:46 pm
content: @NANDBlogMilling out the middle of the board sounds like a really good idea actually... I'll have a think about that and see what kind of copper I can get my hands on. Thank you(I have 10 of the breakout boards to mess with)

author: georges80
author_rank: Frequent Contributor
timestamp: August 03, 2015, 04:52:39 pm
content: Well, the datasheet shows 35C/W for thermal pad soldered and 70C/W without it soldered.Pretty poor thermal performance for a thermal pad device. I'd say, try it (it does have thermal shutdown) without the thermal pad soldered. Doesn't appear to provide that much benefit anyway - at least for a prototype. You could always mount a small heatsink to the top of the chip to 'help a bit'. Use a bit of thermal epoxy or adhesive thermal tape.Hopefully you don't have issues with it on an adapter since it requires some passive components and a crystal to operate.If you want to use the thermal pad, you need to solder to it and aluminium is a no go... Just pressure contact will be pretty useless (folding it etc as you wrote above).cheers,george.

author: MattHollands
author_rank: Frequent Contributor
timestamp: August 03, 2015, 09:21:27 pm
content: Great, I'll have a wackThanks