Circuit Mechanix Dec 2016

Issue 4 December 2016 Circuit Mechanix The magazine for the PCB industry in the UKDesigning reliability inPCB’sHow fabricators canenhance the reliability ofPCB’sReview:OrCAD’s Sigrity ERC toolAssembling reliability intoelectronics.News & more...

Circuit Mechanix Circuit MechanixDecember 2016 –Issue 1 Contents:The PCB magazine forthe UK printed circuitindustry.ContentsArticle:How can engineers design in reliability? P. 3Review:OrCAD Sigrity ERC – Integrating SI Reliability P.6Industry News P. 9Article: P.7The role of fabricators in enhancing thereliability of PCB’sArticle: P. 11Assembling reliability into electronicsPage 1 Circuit Mechanix – December 2016

Circuit MechanixMechanix MusingsIssue four represents a milestone with Circuit Mechanix, it’s now completed it’s first yearof issues. The next issue will be published in 2017 with Spring fast approaching. For nowwe’re looking forward to the depths of Winter and Christmas fast approaching.This issue is looking at reliability, something that whether we like it or not takes in bigpart in the electronics we use in harsh environments such as the automotive electronics werely on working well at this time of year. As designers what do we need to be thinking of?What are the real issues around reliability and how can they be resolved?I would like to hear from you, the issues you have, the problems you can or can’t solve asthis will give me the feedback I need on the real problems you as engineers face so I candirect the content at these areas and who knows – maybe we can all learn something? Ifyou’re interested use the email to get in contact: [email protected], make sure to have a look at the Circuit Mechanix website, sign up by email orbookmark it to keep an eye on what’s going on: https://circuitmechanix.wordpress.com/ Page 2 Circuit Mechanix – December 2016

How can engineers design in reliability?Designing electronics that works on the Of these issues can be reduced orbench is one thing, but lets put these eliminated.electronics in a harsh environments,which is dirty, where the temperature Temperaturechanges from hot to cold, perhaps The first thing to remember aboutrapidly. Maybe the equipment is going temperature changes, is that everythingto be mounted in a helicopter and moves. If warming up then all theexperience extreme vibration or materials expand, when cooling theyperhaps in an environment that subjects contract. If the circuit undergoes rapidthe circuits to steam? heating or cooling or both the board is going to expand and contact. While thisEvery designer should have a kind of isn’t a big issue in the x and y axis ofrisk assessment in their heads of the the board, the z axis is different.factors every design has to face andone of these without a doubt is the The values of thermal expansion ofexpected lifetime of the product or PCB materials is typically greater in theequipment. For example, equipment z axis, but this is widely disregarded asthat only needs to operate only a few the distance is so short. But if a highhours for a race needs to be thought of degree of change takes place then via’sdifferently to the reliability expected for can fracture, often creating a verya board operating on an aircraft without irritating failure that will be hard to findfail for 25 years. and harder to fix.The key to knowing how to design the Reducing the total thickness of theelectronics is to have a good knowledge board can help with this if cheaperof the environment the equipment is materials need to be used. Usinggoing to operate in. What are the materials that are designed to operatetemperatures that it’s expected to in higher temperatures can be veryoperate in? Vibration and mechanical effective as these have a reducedshock are also issues as well as coefficient of thermal expansion.humidity and moisture. The possibilitiescan be daunting – but if a few simples Continued on page 4…things are done, the risks around manyPage 3 Circuit Mechanix – December 2016

How can engineers design in reliability? Mechanical Shock - Vibration… How can engineer’s design in This is perhaps the hardest external factor toreliability - Continued. try and de-risk. Making sure that the pads are large enough, especially for components with aThe extra cost of these enhanced large mass is about all a designer can do. OnlyFR4 materials is often minimal, it when carrying out drop and vibration testingonly get higher when considering can a designer or engineer get an idea of howmuch higher grade materials, the well the electronics will hold together andthis cost is mostly lost in the survive in use.processing of the PCB’s. Humidity & MoistureAnother cost effective solution is toask the fabricator to enhance the However undesirable, humidity and moisturevia wall thickness. This is covered can be a big issue for electronics operatingmore in the next article. outside the home or office. Cars, planes, industrial equipment can all experience this.Lastly the right components need Where heat can create steam, any electronicsfitting to the board. If the subjected to steam are going to have a reallyelectronics are going to be tough time, especially if any other chemicalsoperating in an 80C environment, are in the steam as well. External factors likethen 50C rated components aren’t enclosing the electronics or conformally coatinggoing to be good enough. Make them can be used but add extra cost which cansure the components are rated for be undesirable in some cases. The bestthe task, or it’s likely that alternative is to monitor the moisture the boardsomething weird will start is subjected to, perhaps allowing extra featureshappening at the operating to shut down the circuit or warn the end user.extremes. Continued on page 5…Page 4 Circuit Mechanix – December 2016

How can engineers design in reliability?… How can engineer’s design in Want to see your company name here?reliability - Continued. Contact: [email protected] question is how to do this costeffectively. Adding a conductivetest coupon to act as a sensor toany kind of surface pollution is thefirst step. Adding it to either of thesurface copper layers and leavingit exposed and coated in the samefinish as the rest of the board willcost very little. Coupling this to aspare op-amp and digital I/O couldmean that the only extra expenseis the cost of designing it in andtesting it.Designing in reliability is moreabout thought and understandingthan cost. More can be achievedusing cheap and simpletechniques before spending a lotmore money – it just dependswhere the electronics going andhow important it is that it keeps ongoing.© Circuit Mechanix 2016Page 5 Circuit Mechanix – December 2016

OrCAD’s Sigrity ERC – Integrating Signalintegrity Reliability TextParallel Systems, the UK reseller for The easy to use tool works with theOrCAD and Allegro in the UK are designer to capture and highlight signalhighlighting the Sigrity ERC tool for integrity problems before the PCB isOrCAD. released.Checking electrical rules and See the Sigrity webpage on the Parallelhighlighting signal integrity issues in Systems website:PCB designs “enables the engineer tofind signal integrity issues and aid http://www.parallel-design reliability.” Simon Wood states systems.co.uk/sigrityerc/when discussing Sigrity. Thanks to Simon Wood for permissionSigrity integrates perfectly with OrCAD to use the material.#tools, but it’s all able to work with theother major design tools like PADS, Circuit Mechanix Dec-2016Cadstar and Altium.Page 6 Circuit Mechanix – December 2016

The role of fabricators in enhancing thereliability of PCB’sAs is always discussed, how a PCB PCB laminates, Electro Deposited (ED)fabricator makes the PCB’s that you and Rolled Annealed copper (RA). Bothdesign can make or break a project. processes are self explanatory, butThe same is true with reliability, rolled annealed will typically be moreknowing what to ask and what’s needed robust as it’s a sheet of thinly rolledis most of the battle. If we’re hoping to copper presses onto the laminatecreate a circuit that is going to operate surface. These sheets will also be usedreliably in harsh conditions, what is it when making inner layers on multilayerthat we need to be looking for in our assemblies and bonded between layersfabricator and asking them to do? of prepreg and core materials. It’s not generally known, but a tiny amount ofFinding the higher grade PCB materials lead is added to increase it’s durability,for use in yours boards is all very well – but not enough to break RoHSbut can your PCB fabricator get them directives.and use them? Some of the cuttingedge materials can be very hard to get Electro deposited copper will be morehold of, or can have a long lead time fragile and fractures are more likely toattached to them. Similar products are be made when subjected to mechanicaloften available from a fabricator if your stresses such as thermal expansionneeds are discussed, who knows, and contraction.perhaps even something better.With the materials decided, the circuit The most likely part of a PCB to failhas to be manufactured in a way that when subjected to thermal stress arewill enhance it’s reliability. As always, it the via’s. The z-axis will expand anddepends on the application, but if we’re could fracture via’s. The via wallgoing to consider harsh environments thickness will thin in the centre, creatingand temperature changes the a weakness that can break in thermalconsidering the type of copper will cycling.make a difference.Typically two types are available on Continued on Page 8…Page 7 Circuit Mechanix – December 2016

via’s this isn’t so easy to spot.The role of fabricators in enhancing The condition that needs to be avoidedthe reliability of PCB’s - Continued… is called the keyhole effect (as shown above) where the drill hole is not onA fabricator will typically specify the target and can create a potentialthickness of via wall they’re able to weakness and failure effect where themanufacture. Specifying a minimum via track leaves the bad or via.wall thickness of 25um will reduce therisk of via fracture considerably. There It needs noting that no via hole will everis no way to eliminate this completely of be spot on – but it needs to be withincourse especially in cases of extreme the stated manufacturing tolerances oftemperature change. the fabricator. The IPC-601 *** standard will give guidance on the acceptabilityThe quality and accuracy of via drilling criteria of this an many otheralso has a distinct affect on via manufacturing defects that may arisereliability. Drill bits need to be replaced from a fabricator. It will also give ana regular intervals to ensure the drill is idea of the acceptability criteria tosharp when every via is drilled. A blunt specify when asking them to supplydrill will create a rough via wall that boards.could fail, especially when stressed.Drill accuracy is how on target every via Circuit Mechanix © 2016has been drilled. On larger via’s anpads it’s easy to see this, but on smallerPage 8 Circuit Mechanix – December 2016

Circuit MechanixIndustry News: Options are now availablPage 9 Circuit Mechanix – December 2016

Assembling reliability into electronicsMaking reliable electronic assemblies is In different parts of the PCB can createabout making a good solder joint isn’t problems in achieving a good result.it? Using a longer thermal ramp up andSo what’s the problem, plop down some dwell time can often help but gettingpaste in the right area place the heat under leadless components andcomponent carefully down, apply heat BGA’s will always create difficulties.and the solder melts. Hey presto! Dry joints and voids under theseYes, if only it was that easy. The Jedec components will not be immediatelythermal profile is well known and was obvious with only x-ray inspection ablementioned in a previous issue???? The to reveal any issues. Many BGA’s haveproblem is every part of a PCB will have hundreds of balls on them with everya different thermal profile as will every singe one needing to be perfectlycomponent and joint. In the ideal world soldered in order to operate properlythe flux needs to activate at the same and reliably.time and the solder melt at the sametime, but this is never going to happen. Vapour phase reflow is becoming more popular in reflowing modern highThe result of this is an electronic density PCB’s as this gives a far moreassembly can suffer from dry joints in uniform and reliable thermal profileplaces and others voids, head in pad or across the PCB being assembled, evenpossibly tombstoning. It’s quite a for joints under the component such asjuggling act that needs to be performed. BGA’s. It tends to be a slower way ofUnfortunately it’s mainly trial an error, reflowing boards, so it isn’t used asbut knowing what to consider before much in higher quantities, but thegoing to assembly can help avoid some process gives good results across theof these nasty niggly errors. board.It’s more difficult to uniformly heat a Continued on Page 11…PCB to the correct Jedec profile.Thermal shadows and hotspotsPage 10 Circuit Mechanix – December 2016

Assembling reliability into electronicsAssembly and Rework of Multilayer leadless component to protect the jointsPCB’s continued… and give extra mechanical rigidity.So now we know how to make a good Conformal coating are sprayed over asolder joint that’s it right? The board PCB to protect the board from moisturecan be put into service and will work for and other contaminants. There areyears – problem solved? many types to use and choosing the right one to protect against theNo. contaminants it’s expected to encounter will need careful consideration.Many PCB’s are put into service inenvironments that are harsh, with high Both of these processes should only belevels of moisture, dust, or some kind of applied to a tested working board. Ifchemical pollution. Any one of these will these are to be used on a cleanedaffect the long term operation of the board, the board must be really cleanproduct by either forcing the board to as any residue flux is likely to corrodeoverheat or causing problems in the any joints or copper over time (such isoperation of the circuit. the nature of no clean fluxes). Once a conformal coating has been applied,Using underfills for BGA’s and a there’s little that can be changed on aconformal coating can help protect a board, so it’s make or break. If all thiscircuit from these effects, but a works, your PCB will work in somemaintenance schedule could still be harsh environments for a good timeneeded to clean the boards. without trouble.An underfill is a liquid that fills in under Circuit Mechanix © 2016a BGA or otherInterested in seeing your company name in here, or doing something you want to tell everyone about?Email [email protected] for more information.Page 11 Circuit Mechanix – December 2016

Circuit Mechanix Next Issue: Next issue out in March 2017 - What’s going to be in it? Articles: Designing flexible circuits effectively Flexible Circuits – Getting them fabricated and how they differ from rigid PCB’s Assembling flexi’s and flex rigid circuits Review: Web based PCB resources. And more…Page 12 Circuit Mechanix – December 2016