X-Authentication-Warning: delorie.com: mail set sender to geda-user-bounces using -f X-Recipient: geda-user AT delorie DOT com X-Amavis-Modified: Mail body modified (using disclaimer) - mta-out02.sim.rediris.es Date: Wed, 10 May 2023 16:04:01 +0200 From: "Gabriel Paubert (paubert AT iram DOT es) [via geda-user AT delorie DOT com]" To: "Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com]" Subject: Re: [geda-user] [off-topic] Large resistance value SMD resistors? Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: X-Proofpoint-ORIG-GUID: 9iAF8IWVTUkap54eAQqplnGPUH1U9iC_ X-Proofpoint-GUID: 9iAF8IWVTUkap54eAQqplnGPUH1U9iC_ X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.254,Aquarius:18.0.942,Hydra:6.0.573,FMLib:17.11.170.22 definitions=2023-05-10_04,2023-05-05_01,2023-02-09_01 X-Proofpoint-Spam-Details: rule=outbounddefault_notspam policy=outbounddefault score=0 malwarescore=0 suspectscore=0 adultscore=0 spamscore=0 mlxscore=0 clxscore=1015 bulkscore=0 phishscore=0 priorityscore=1501 impostorscore=0 mlxlogscore=960 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2304280000 definitions=main-2305100112 Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by delorie.com id 34AE4fFv026002 Reply-To: geda-user AT delorie DOT com Errors-To: nobody AT delorie DOT com X-Mailing-List: geda-user AT delorie DOT com X-Unsubscribes-To: listserv AT delorie DOT com Precedence: bulk Hi Richard, On Wed, May 10, 2023 at 01:10:55PM +0200, Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com] wrote: > Hi all, > > My apologies for this off-topic question, but I'm running into a question > for which I can't find a good answer online -- and I know that some people > here have special expertise > > I'm asked to evaluate another tech's design, and apart from the inevitable > small optimization things, he did a fine job. There is however one thing I'm > not sure about: the circuit features a couple of 22 megohm resistors, and > the current choice is simply a 0603 size. As the device in question may be > used in environments without proper climate control, I think that moisture > (condensation) may on occasion become a problem given the short insulation > distance. All voltages are 15 volts and lower, so there's no hazard > involved, but the circuit's specifications are no longer met if the > resistance drops below ~15 megohms. > > Would it be better to use a larger resistor (e.g. 1206), or forego SMD > altogether? I can of course also specify a moisture-resistant PCB coating, > but that is a rather expensive extra production step. > Isn't it cheaper to put a encapsulate the resistor and all the solder joint area in expoxy glue after the assembly (and testing)? I would consider products like: https://epotekeurope.com/wp-content/uploads/2022/08/EPO-TEK%C2%AE-Adhesives-Applications-%E2%80%94-PCB-View.pdf page 11 (potting compounds). And yes, it will be difficult oimpossible to replace the resistor if it fails, but the power dissipated by a 22MΩ resitor with less than 15V across is so small that it should never be a problem in practice. I'm not in any way affiliated with Epoxy Technology, and there are other manufacturers. I've just used their products: H20E silver based conductive epoxy to attach (ground coonection) backside of bare chips and 360T to hold conical inductors like: https://piconics.com/wp-content/uploads/2019/07/G5-Series-Low-Current.pdf For the record, I bought half a pound (sorry for the units) of 360T in January 2022, delivered in March, for a bit less than €200. With that you can probably encapsulate thousands of 603. The stated shelf life (in theory 6 months for 360T), and 1 year for most epoxies, is from my experience very conservative. I've just hace a look and my 360T still looks fine, will try to use it in a few weeks if all goes according to plans. HTH, Gabriel > Thanks in advance for any ideas, > > Richard Rasker >