Tips on Cooking Both a Perfect Thanksgiving Turkey and a PCB

Why does it take half a day to cook a Thanksgiving turkey?  The answer is simple ― you have 20 lb of bird that simply cannot just be nuked in a microwave like last night’s dinner.  If not properly thawed, prepared and monitored, you either have an overcooked, dried-out bird or worse: Salmonella. Strangely enough, as you will see in a moment, PCBs are not that much different.

Let’s say you skip the thawing process and in your haste stick a frozen bird in the oven.  What happens?  The bird may look properly cooked on the outside, but as soon as you try your skill with the carving knife, you either hit bedrock or the inside is completely raw. OK, I will admit I speak from personal experience on this one (please do not bring this up with my wife).  Are PCBs any different?  Well, your reflow profile has a preheat phase, with the purpose of bringing your PCB to temperature. In other words, the entire mass of the board with all its components is gradually brought to equilibrium. If you do not do this, you run the risk of thermally shocking your components when they hit reflow and peak.  Thawing your bird and preheating your PCB ― you have the same objective in mind.

So, for the vast majority of us, we really have no idea when the turkey is fully cooked until getting an internal reading. A PCB is no different. On the surface, both might look great, but upon closer inspection, you discover some components have defects due to improper reflow or, for that matter, when you cut into a turkey that is still pink it really hits home that you aren’t cooking a TV dinner.

turkey-in-Spec_SM01

Because of this, as we all know, a 20 lb turkey requires a thermometer. I will concede that some of you use the old “poke the bird and check for pink until done” trick. Let’s assume you are not as skilled, like me, for example. Would you seriously cook a turkey by relying solely on the oven’s temperature reading on your stovetop?  Of course not, but why do some of you profile your PCB by relying on your reflow oven’s reported readings? Are either situation that much different?  Actually, yes. Your nice self-contained turkey cooking oven is more of a steady state, but there remains a large difference between what is reported by the oven and the internal temperature of your turkey. In contrast, your PCB is exposed to anything but a steady state environment because it rides on a conveyor through different heated zones with blowers, extraction systems and both ends of the oven even open to the elements!  For this reason, any oven manufacturer will adamantly tell you to profile and with regularity. Alright, you may have learned how to cook a turkey in your Mama’s kitchen and, in fact, be skilled at not using a thermometer; however, I doubt any serious SMT manufacturer would take a similar approach, checking your PCBs regularly for “doneness” in your reflow process.

What about placing the fate of your Thanksgiving feast on the cheap-o plastic pop-up indicator that likely came with the turkey? Do not laugh. How many of us use the trailing wires that came with the reflow oven?  Now to be fair, both work in principal; otherwise, you would have the likes of Purdue Farms with food poisoning lawsuits on their hands, but they only give you ballpark readings in many cases. By design, the turkey is going to be a little overdone and dried out.  Your PCB, on the other hand, cannot afford to be a little overdone or it is simply OUT of spec.  You can get by with eating the overcooked turkey … the gravy and mashed potatoes are there to make up for less than a perfectly cooked bird. But your PCB will not be as forgiving.  Trailing wires, never mind being cumbersome to use, have a tendency to kink and stretch, which compromise their readings.  They also are susceptible to 50 or 60 cycle noise from some reflow oven environments, further questioning their accuracy in some cases.

So you want to cook the perfect bird. Who doesn’t? So you pony up for a stainless steel large-dial meat thermometer to accurately read the internal temperature of your 20 lb bird. You also pony up for a KIC Explorer with Navigator because you want to create the perfect deep-in spec reflow profile. It will not only tell you the specific temperature of the joints of your $500 BGAs, but it also will find a balance that does not overcook them or any of your other temperature-sensitive components on the PCB.  No pop-up indicator profiler needs to apply since the KIC Explorer with Navigator will go the extra mile and tell you not only if you are in-spec but how DEEP in-spec your profile is, along with what can you do to improve the profile in minutes, if not seconds.  Now do you know of any turkey thermometers that can do that?

So when you prepare your Thanksgiving turkey, and as you pause to give thanks, consider applying the same care and consideration that you have given to your family’s feast as you do to your PCBs.

Happy Thanksgiving – Profilingguru

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Reducing Reflow Product Changeover Time

2009 Presentation at SMT Long Island on how to reduce the changeover time from one reflow profile recipe to another.  If you ever opened up your reflow oven to dump all its heat to lessen downtime, this 4 min video is for you!!!

To view the complete video series (click here).

To subscribe to my Podcast for iTunes (click here).

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SMT related Links to know

RSS feeds, Tweets, blogs and newsletters, how do you keep up?   Well here is the latest on what’s available in the SMT industry.   I subscribe to all of these newsletters and regularly pick out areas of interest related to profiling for you.   I also comb the blogs though I only know of two, not including profilingguru, which is quite remarkable considering other industries have hundreds if not thousands.   The SMTA group forum on LinkedIn yields on occasion a nugget, but you need to build a profile to join.  SMTnet has always been a jewel.  Lastly, Twitter is a new phenomenon for many of us.   I am still trying to get the knack of it myself but it does have some value no doubt and will continue to grow.

On-line Newsletters:

Circuitnet

Electronics Production World

EMS Now

GlobalSMT

PCB Update

SMT Week

Blogs:

Circuits Assembly

Forums:

SMTA on LinkedIn

SMTnet

Twitter:

Circuit Assembly

Global SMT

SMT Magazine

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Profiling guidelines for reflow of solder bump flip chip attach to organic and/or ceramic packages

This post is in response to a suggested topic on this blog.  The following answer was provided by Brian J. Toleno, Ph.D., Director Technical Service at Henkel:

When profiling a flip-chip to an organic substrate you typically want the delta T across the component to be as low as possible in order to minimize stresses and warpage. Of course, like any good profile, making sure that the solder bumps and solder paste reach liquidus, stay above the liquidus temperature for the recommended time, and have a controlled cool down as possible are key.   In addition, if the flip chip device is going to be underfilled, then it is important that when using a no-clean flux solder paste that the solder paste is fully activated in order to minimize any possible flux/underfill interactions.    So making sure you measure the temperature at the flip-chip bump/solder paste/solder pad area is important.   You also want to make sure you measure the temperature at the centre and the corners (making sure they track close together).   While there typically is not as great of a chance of a large delta T, like there would be in a BGA or CSP device – when one does occur it can be more catastrophic.

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Components

Technician fitting a processor

Paste specs will typically exceed those of components, therefore, much thought is typically not given to component specs until they are literally flying off your PCBs.  Like PCBs, the material of the components plays a role.  I’ve seen more time and effort spent on profiling ceramic packages than I care to remember. When you have a capacitor close to a large ceramic BGA, how do you get the BGA to reflow and still keep the capacitor from looking like popcorn?  One common technique is to isolate each component with its own individual process spec and then run this new setting through the profiling software prediction, as discussed in this prior.  The profiling software will tell you if it is possible to achieve what you want with your given board, paste and oven. Solutions to shield sensitive components and a complete re-design may also be in order. Additionally, the solution might be that you need new equipment that can follow the new process recommendations, allowing for tighter controls and/or more zones.  Your profiler will calculate the possibilities.

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Setting up your Solder Spec

Many profiling software packages will have a pre-loaded library of solder specs. You should still refer to the solder paste manufacturer’s data sheet to confirm accuracy, but these libraries are becoming ever more accurate and robust. Providers, such as KIC, offer regular updates online for download.


solderpastemenu

Solder Paste Library from KIC® 2000 software.

Once you input your solder paste spec, you can use the default setting or further refine your solder spec to reflect limitations to component temperatures and change other parameters as well.

solderpastepeak

You can further refine any of your paste specs to compensate for component tolerances and other inputs specific to your process.

For example, your paste may allow a certain maximum peak temperature, but knowing that one of your components cannot tolerate such a high temperature, you can manually change a peak of 245°C to the component’s tolerance of 243°C.

Many profiling software packages also allow you to define specific segments of your profile.  You may, for example, have more than one rising slope.

solderpasteinputs

Many inputs can be defined for your profile.

Lastly, you may need to define component or density-specific regions of your PCB.  You might first define a general profile for the entire PCB and then pull out focused areas on the PCB with tighter or expanded specs.  For example, you may need to tighten your temperature tolerance for ceramic packages. Tighten the specs around a troubled area, while loosening them elsewhere.  After all, your goal is to produce a PCB in spec.  Some profiling software packages can help you target these areas, even TC-specific.

solderpastetcs

Each TC can have its own individual spec, which is common when you have a component which high heat tolerance issues.

You can eliminate wasted time and effort by simply running “what if” scenarios.  In the case of PWI-driven software, a single value is assigned to your user-defined inputs that indicates if a process and/or component are within spec. By tracking these PWI values alone, instead of trying to interpret hundreds of data points, a multi-variant statistical relationship is created for all inputs. A single value, or PWI, is outputted. The bottom line is that you will get your process within spec, with the least amount of effort!

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