NPL Soldering Defects Database

NPL Defects Database

The Soldering Defect Database is a freely accessible resource from the National Physics Laboratory website:

Defects Database

This little known resource is a fully searchable database of soldering defects with pictures, and information about the causes and solutions of each problem. Once signed up not only is all the information available but the database can be added to with any findings a user wishes to add.

This is a truly a brilliant resource and I can’t think of how many occasions this could have been of use in the past. Have a look, get involved and make sure to pass it onto your colleagues.

Circuit Mechanix Apr-2017

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The PCB Mechanic – The UK PCB industry and facing the challenges of the future

PCB Mechanic

 

PCB fabricators in the UK are facing the challenges of being able to make the PCB’s that the designers need to accommodate these modern packages. This is no easy challenge, fabricators based in Asia and other European countries are providing their customers with advanced capabilities that not long ago were not possible. Not only do these fabricators have the capability, but the manufacturing costs cannot be matched equivalent fabricators in the UK.

Electronics are advancing at an ever increasing rate and this rate has no sign of abating. Integrated circuits are being developed all the time with increasing speeds, functionality and higher densities than were available before.
This gives engineers the components they need to create products with more functionality in a much smaller form factor than has been possible before. No other market shows more evidence of this than the smart phone and tablet market. These supercomputers in our pockets were inconceivable in such a compact size fifteen years ago.

Chip packages come with far more, smaller connections, whether they’re balls, pads or pins in a much more compact package. In for UK fabricators to win business to serve this insatiable demand for smaller and better electronics, they need to be able make the boards that are being designed today and be looking forward to the designs needed in the years to come. It doesn’t seem that long ago that a track and gap of less than 0.2mm would be the exception rather than the norm, but modern electronics has ended this forever.

It seems however that many UK fabricators are struggling to realise this and are not investing enough to keep up with the rate of change.

The high density PCB packages of today are forcing designers to use track and gaps of less than 0.1mm or copper filed via in pad, or via sizes of 0.1mm just to be able to route connections out to the rest of the circuit. BGA’s aren’t the only issue, dual row and other leadless packages where the pins are 0.5mm apart or less are forcing designers to make tough choices about where they place the compromise in their designs.

BGA’s aren’t the only issue, dual row and other leadless packages where the pins are 0.5mm apart or less are forcing designers to make tough choices about where they place the compromise in their designs.

There aren’t many PCB fabricators left in the UK and only a handful are able to compete with the capabilities that offshore companies are able to give. The reality is that there is a tough job ahead to catch up and keep up and many PCB fabricators haven’t advanced their capabilities in over 10 years. This is an age in technology terms. The UK cannot possibly compete on cost with offshore manufacturing, but if they can’t do at least the same or better, there is no way to compete with the offshore fabricators.

Simon Farnell The PCB Mechanic

This article was features in the March ‘What’s new in Electronics’ newsletter:
http://wnie.co.uk/library/uk-pcb-industry-facing-electronics-challenges-future/

Flexi PCB’s and making something wobbly like it’s not

Just like standard rigid PCB’s flexi’s and flex rigid PCB’s have to have components assembled onto them to make them useful in a circuit. However unlike rigid PCB’s flexi’s are well… flexible.

The flexible nature of a flexi is it’s strength in the field, but during manufacture causes nothing but problems! There are ways and means around this and that’s what’s going to be discussed here.

Part of the issue is that the fabricator needs to make the FPCB’s on a rigid panel that the assembler can accept and work with. In prototyping volumes this isn’t likely to be an issue, but if thousands or millions of FPB’s are to be made then there has to be a solid working interface between the fabricator and assembler to make it work optimally.

PCB’s are usually more efficient to assemble in larger volumes on a panel. Flex panels tend to have less circuits on them and a smaller as the panel, even with stiffeners are often weaker. If there are components on both side of an FPCB, then making the panel so that solder paste can be deposited onto the board also needs considering. Often stiffeners or panels sit above the FPCB, making paste deposition impossible. It’s for this reason that good communication is needed between designer and fabricator to get every detail right

Be warned – making a the same design in different companies can result in different approaches and things going wrong. Because there is so much more for the manufacturer to understand than rigid PCB’s like stiffeners, panels etc, there is more to get wrong.

Layer stackups are also something to watch out for, especially between different fabricators and sometimes different factories within the same company. The differences are often very minor and irrelevant but even small differences can have an effect on a design – especially in high speed circuits.

Good fabricators will highlight these changes and give their best alternative so keeping track of what’s being built can be done.

The nightmare every PCB designer needs to consider in FPCB’s is how any components will be assembled onto the board. If the flexi is on a panel this is a good first step, areas where components need to be assembled onto them will need to be secured or assembly will be impossible.

Fabricators will have tape or some other kind of way to secure
Component area’s on FPCB’s. Identify these area’s in the output data and label them to make it easier for the fabricator to identify these areas and process them accordingly. Doing this will make assembly far easier – the only problem after this might be peeling the flexi away from the panel without damaging the components or their solder joints.

It’s for this reason that assembling components onto FPCB’s should only be done when it absolutely has to be done.

If you’re not putt off yet, you should be – no one said it would be easy!

Circuit Mechanix © 2016

Designing Flexi’s and Flex Rigids – What’s Involved?

Flexible and flex rigid PCB’s are offering solutions to product designers that are not just a luxury in this day and age but a necessity. The technology itself isn’t that new, flex PCB’s have been around for over 30 years. But in today’s world of extremely compact and high speed connections often there is no choice but to use them over conventional electronic assemblies using wired connections to connect between PCB’s and these are to large. This is the main reason for their increased use in the last 10 to 15 years.

But how does the PCB designer go about designing a flexi or flex rigid PCB with components on it and how can the design be made easier to manufacture?
The basic Do’s and don’ts around flexi circuits we can get a picture of what’s involved and why:

a) To avoid the flex material tearing in manufacture or use, put as large a radius as possible on all internal corners.

FCB1

b) Via’s should not be placed in bend area’s as they can crack.

FCB3

c) Tracks running through bend areas should be routed at 90º to the bend.

d) Tracks on flexi’s ideally should be routed with filleted corners to stop them breaking during flexing.

e) Copper pours on flexi’s should ideally be hatched, especially in bend area’s. Because of stresses in the solid copper pour fractures are likely especially during flexing.

FCB2

This simple guide gives the designer a good basis on which to design flexi PCB’S (FPCBs) well. The challenges don’t end here though. Ideally an FPCB will have stiffeners under areas where components are going to be in the circuit. You can bet there will be times where some bright spark will decide for one of many reasons that the FPCB will have components and no stiffeners.

This is not an easy task and the designer is likely to get the task of working out how this can be done. This usually means working out what kind of stiffening frame can be used with the fabricator and assembler and sticking the FPCB down.

The reason for sticking down and FPCB for assembly is clear when it’s understood that the copper on an FPCB is giving at least as much structural rigidity as the substrate itself. Stresses in the copper push and pull the shape into interesting and often unwanted shapes. Assembly would be impossible without sticking the FPCB down to something rigid.

Added to this the copper finish isn’t very durable and can crack if flexed. Keeping these component pads away from bend area’s is necessary to ensure they survive through to assembly, but if it putting the component in a bend area can’t be helped then sticking the FPCB down to secure it is another reason why sticking it down is not be a bad approach to use.

As always it’s never a bad thing to engage manufacturer’s in the design stage and with flex this is even more important, even if only at first until all of the design aspects are better understood.

© Circuit Mechanix 2017

 

Circuit Mechanix – March 2017

 

Circuit Mechanix Mar 2017 - Front Cover

Welcome to the first issue of Circuit Mechanix this year! A little later than expected this issue focuses on flexi and flex rigid PCB design and manufacture.

 

This issue also looks at the National Physics Laboratory soldering defects database  and we get a new feature – The PCB Mechanic. Every issue will now feature some of his thought on what’s going on.

 

 

 

 

Have a look – get in touch and get involved if you like. This is a young project and help with news and features is needed.

Download the PDF for the magazine here:

Circuit Mechanix Mar 2017

I’m still not able to provide a flip book – as soon as I get over this technical hurdle I will let you all know!

There is also a LinkedIn Group for the Magazine and discussion around it here:

Circuit Mechanix LinkedIn Group

Also have a look at the Circuit Mechanix Facebook Page and follow from there!

Assembling reliability into electronics

Making reliable electronic assemblies is about making a good solder joint isn’t it?

So what’s the problem, plop down some paste in the right area place the component carefully down, apply heat and the solder melts. Hey presto!

Yes, if only it was that easy. The Jedec thermal profile is well known and was mentioned in a previous issue???? The problem is every part of a PCB will have a different thermal profile as will every component and joint. In the ideal world the flux needs to activate at the same time and the solder melt at the same time, but this is never going to happen.

The result of this is an electronic assembly can suffer from dry joints in places and others voids, head in pad or possibly tombstoning. It’s quite a juggling act that needs to be performed. Unfortunately it’s mainly trial an error, but knowing what to consider before going to assembly can help avoid some of these nasty niggly errors.

It’s more difficult to uniformly heat a PCB to the correct Jedec profile. Thermal shadows and hotspots in different parts of the PCB can create problems in achieving a good result. Using a longer thermal ramp up and dwell time can often help but getting heat under leadless components and BGA’s will always create difficulties.

Dry joints and voids under these components will not be immediately obvious with only x-ray inspection able to reveal any issues. Many BGA’s have hundreds of balls on them with every singe one needing to be perfectly soldered in order to operate properly and reliably.

Vapour phase reflow is becoming more popular in reflowing modern high density PCB’s as this gives a far more uniform and reliable thermal profile across the PCB being assembled, even for joints under the component such as BGA’s. It tends to be a slower way of reflowing boards, so it isn’t used as much in higher quantities, but the process gives good results across the board.

 

So now we know how to make a good solder joint that’s it right? The board can be put into service and will work for years – problem solved?

No.

Many PCB’s are put into service in environments that are harsh, with high levels of moisture, dust, or some kind of chemical pollution. Any one of these will affect the long term operation of the product by either forcing the board to overheat or causing problems in the operation of the circuit.

Using underfills for BGA’s and a conformal coating can help protect a circuit from these effects, but a maintenance schedule could still be needed to clean the boards.

An underfill is a liquid that fills in under a BGA or other leadless component to protect the joints and give extra mechanical rigidity.

Conformal coating are sprayed over a PCB to protect the board from moisture and other contaminants. There are many types to use and choosing the right one to protect against the contaminants it’s expected to encounter will need careful consideration.

Both of these processes should only be applied to a tested working board. If these are to be used on a cleaned board, the board must be really clean as any residue flux is likely to corrode any joints or copper over time (such is the nature of no clean fluxes). Once a conformal coating has been applied, there’s little that can be changed on a board, so it’s make or break. If all this works, your PCB will work in some harsh environments for a good time without trouble.

Circuit Mechanix © 2016

OrCAD’s Sigrity ERC – Integrating Signal integrity Reliability

cadence-sigrity

Parallel Systems, the UK reseller for OrCAD and Allegro in the UK are highlighting the Sigrity ERC tool for OrCAD.

Checking electrical rules and highlighting signal integrity issues in PCB designs “enables the engineer to find signal integrity issues and aid design reliability.” Simon Wood states when discussing Sigrity.

Sigrity integrates perfectly with OrCAD tools, but it’s all able to work with the other major design tools like PADS, Cadstar and Altium.

The easy to use tool works with the designer to capture and highlight signal integrity problems before the PCB is released.

See the Sigrity webpage on the Parallel Systems website:

http://www.parallel-systems.co.uk/sigrityerc/

Thanks to Simon Wood for permission to use the material.

Circuit Mechanix Dec-2016

The role of fabricators in enhancing the reliability of PCB’s

As is always discussed, how a PCB fabricator makes the PCB’s  that you design can make or break a project. The same is true with reliability, knowing what to ask and what’s needed is most of the battle. If we’re hoping to create a circuit that is going to operate reliably in harsh conditions, what is it that we need to be looking for in our fabricator and asking them to do?

Finding the higher grade PCB materials for use in yours boards is all very well – but can your PCB fabricator get them and use them? Some of the cutting edge materials can be very hard to get hold of, or can have a long lead time attached to them. Similar products are often available from a fabricator if your needs are discussed, who knows, perhaps even something better.

With the materials decided, the circuit has to be manufactured in a way that will enhance it’s reliability. As always, it depends on the application, but if we’re going to consider harsh environments and temperature changes the considering the type of copper will make a difference.

Typically two types are available on PCB laminates, Electro Deposited (ED) and Rolled Annealed copper (RA). Both processes are self explanatory, but rolled annealed will typically be more robust as it’s a sheet of thinly rolled copper presses onto the laminate surface. These sheets will also be used when making inner layers on multilayer assemblies and bonded between layers of prepreg and core materials. It’s not generally known, but a tiny amount of lead is added to increase it’s durability, but not enough to break RoHS directives.

Electro deposited copper will be more fragile and fractures are more likely to be made when subjected to mechanical stresses such as thermal expansion and contraction.

The most likely part of a PCB to fail when subjected to thermal stress are the via’s. The z-axis will expand and could fracture via’s. The via wall thickness will thin in the centre, creating a weakness that can break in thermal cycling.

A fabricator will typically specify the thickness of via wall they’re able to manufacture. Specifying a minimum via wall thickness of 25um will reduce the risk of via fracture considerably. There is no way to eliminate this completely of course especially in cases of extreme temperature change.

The quality and accuracy of via drilling also has a distinct affect on via reliability. Drill bits need to be replaced a regular intervals to ensure the drill is sharp when every via is drilled. A blunt drill will create a rough via wall that could fail, especially when stressed.

Drill accuracy is how on target every via has been drilled. On larger via’s an pads it’s easy to see this, but on smaller.

The condition that needs to be avoided is called the keyhole effect (as shown above) where the drill hole is not on target and can create a potential weakness and failure effect where the track leaves the bad or via.

It needs noting that no via hole will ever be spot on – but it needs to be within the stated manufacturing tolerances of the fabricator. The IPC-A-600 standard will give guidance on the acceptability criteria of this an many other manufacturing defects that may arise from a fabricator. It will also give an idea of the acceptability criteria to specify when asking them to supply boards.

Circuit Mechanix © 2016

How can engineers design in reliability?

Designing electronics that works on the bench is one thing, but lets put these electronics in a harsh environments, which is dirty, where the temperature changes from hot to cold, perhaps rapidly. Maybe the equipment is going to be mounted in a helicopter and experience extreme vibration or perhaps in an environment that subjects the circuits to steam?

Every designer should have a kind of risk assessment in their heads of the factors every design has to face and one of these without a doubt is the expected lifetime of the product or equipment. For example, equipment that only needs to operate only a few hours for a race needs to be thought of differently to the reliability expected for a board operating on an aircraft without fail for 25 years.

The key to knowing how to design the electronics is to have a good knowledge of the environment the equipment is going to operate in. What are the temperatures that it’s expected to operate in? Vibration and mechanical shock are also issues as well as humidity and moisture. The possibilities can be daunting – but if a few simple things are done, the risks around many

Of these issues can be reduced or eliminated.

Temperature

The first thing to remember about temperature changes, is that everything moves. If warming up then all the materials expand, when cooling they contract. If the circuit  undergoes rapid heating or cooling or both the board is going to expand and contact. While this isn’t a big issue in the x and y axis of the board, the z axis is different.

The values of thermal expansion of PCB materials is typically greater in the z axis, but this is widely disregarded as the distance is so short. But if a high degree of change takes place then via’s can fracture, often creating a very irritating failure that will be hard to find and harder to fix.

Reducing the total thickness of the board can help with this if cheaper materials need to be used. Using materials that are designed to operate in higher temperatures can be very effective as these have a reduced coefficient of thermal expansion. The extra cost of these enhanced FR4 materials is often minimal, it only get higher when considering much higher grade materials, the this cost is mostly lost in the processing of the PCB’s.

Another cost effective solution is to ask the fabricator to enhance the via wall thickness. This is covered more in the next article.

Lastly the right components need fitting to the board. If the electronics are going to be operating in an 80C environment, then 50C rated components aren’t going to be good enough. Make sure the components are rated for the task, or it’s likely that something weird will start happening at the operating extremes.

Mechanical Shock – Vibration

This is perhaps the hardest external factor to try and de-risk. Making sure that the pads are large enough, especially for components with a large mass is about all a designer can do. Only when carrying out drop and vibration testing can a designer or engineer get an idea of how well the electronics will hold together and survive in use.

Humidity & Moisture

However undesirable, humidity and moisture can be a big issue for electronics operating outside the home or office. Cars, planes, industrial equipment can all experience this. Where heat can create steam, any electronics subjected to steam are going to have a really tough time, especially if any other chemicals are in the steam as well. External factors like enclosing the electronics or conformally coating them can be used but add extra cost which can be undesirable in some cases. The best alternative is to monitor the moisture the board is subjected to, perhaps allowing extra features to shut down the circuit or warn the end user.

The question is how to do this cost effectively. Adding a conductive test coupon to act as a sensor to any kind of surface pollution is the first step. Adding it to either of the surface copper layers and leaving it exposed and coated in the same finish as the rest of the board will cost very little. Coupling this to a spare op-amp and digital I/O could mean that the only extra expense is the cost of designing it in and testing it.

Designing in reliability is more about thought and understanding than cost. More can be achieved using cheap and simple techniques before spending a lot more money – it just depends where the electronics going and how important it is that it keeps on going.

© Circuit Mechanix 2016

Contact: circuitmechanix@gmail.com

 

Circuit Mechanix December 2016

 

circuit-mechanix-dec-2016

 

Welcome to the fourth and last issue of Circuit Mechanix for 2016.

In this month’s issue the theme of reliability in electronics is looked at and how this can be addressed at each stage of the PCB design and manufacturing process

This issue also has a look at how OrCAD’s Sigrity can assist in design reliability.

Have a look – get in touch and get involved if you like. This is a young project and help with news and features is needed.

 

 

Download the PDF for the magazine here:

circuit-mechanix-dec-2016

I’m still not able to provide a flip book – as soon as I get over this technical hurdle I will let you all know!

There is also a LinkedIn Group for the Magazine and discussion around it here:

Circuit Mechanix LinkedIn Group

Circuit Mechanix – A mazagine for the UK based printed circuit Industry.