Reducing Reflow Oven Power Consumption – RIT Study

In a study lead by Prof. S. Manian Ramkumar of Rochester Institute of Technology, a new minimize energy consumption featured available with KIC Navigator was studied to see if when producing new profiles for existing processes can yield energy savings while maintaining an in spec process.

(for the full study, download to pdf here).

Three different companies were selected for this project.

  • Company A:  Surmotech CMS is a full-service contract manufacturer specializing in high-reliability medical, industrial and military applications.  Located in Victor, NY, Surmotech is an ISO 9001:2000 registered company that focuses on providing complete turnkey solutions that include design, prototyping, engineering, materials management, testing and field service.
  • Company B: Marquardt is a global manufacturer of electromechanical and electronic components, supplying the hand tool and automotive industries.
  • Company C: SenDEC is organized into two main business units: the Contract Electronics Manufacturing (CEM) Group and the Products Group.  SenDEC’s CEM Group provides electronics manufacturing services (EMS) including design, prototype, PCB assembly, electromechanical assembly, test engineering, rework, material management and turnkey box build services.  The SenDEC Products Group manufactures its own family of digital monitoring, display and control devices for numerous markets across the globe.

Conclusions of the Study:

  • The recipes recommended by the new KIC Navigator feature have been observed to have either reduced the power consumption when compared existing recipe or maintained it the same.

powerconsumption1

  • Furthermore, it is also observed that in most cases, the percentage reduction in power consumption is 2% or higher, as shown in the figure below.

powerconsumption2

  • The new Navigator Power option has the capability to generate a reduced power consumption recipe without compromising the productivity and the quality of the output.
  • The software provides the flexibility to incorporate the restrictions offered by the reflow oven.
  • The updated Profiling software version has a large solder paste menu (both Sn-Pb and Pb-Free) to select from.  This feature automatically feeds the solder paste specifications.
Share

Common Defects

Bridging is characterized by solder unintentionally attaching to adjacent pads.  Bridging is typically a print issue, but can be caused by ramp rates that are too slow. A possible solution is to increase the ramp rate to 2.5-3°C/sec and a 150°C soak. Obviously, the down side is that you will have less activity when the paste reaches reflow.

defectsInsufficient Wetting (Dry Lead Toe & Heels, Poor Fillets) occurs when solder does not flow fully into a joint, typically as a result of too short of a soak (insufficient cleaning time) or too-long of a soak (all flux cleaners are used up and new oxides form). The main purpose of the flux in the solder paste is to prep the surfaces in order to create a good solder joint. If the activator (flux) is burned off too quickly, you will see wetting problems. Ramp can also factor into poor wetting.

Microcracks occur at the interface between ball and pad when incompletely formed.  This may be caused from too little time at reflow or too low of peak temperatures (head-in-pillow). Microcracks are also associated with moisture in your components, similar to the causes associated with popcorning.

Popcorning is when components (especially ICs) break open due to expansion of trapped moisture, typically because the ramp rate is too fast. Controlling your ramp rate through 100°C and improving your storage conditions are possible solutions.

Solder Balls are formed from oxidized solder paste that fail to coalesce from ramp rates that are either too slow or too fast.  In the case of a ramp rate that is too fast, volatiles in the paste are driven off before the solder becomes molten.   Solder balls can also be caused by letting the paste sit too long on the PCBs before reflow.

Solder Beading should not be confused with solder balls. Solder Beading is really a printer issue, and not directly attributed to reflow. Solder Beading is typically caused by no clean solder pastes. In this case, the apertures have not been reduced in the stencil and there is too much paste volume.  The paste is actually squeezed out under discrete parts and creates a mid-chip ball that appears to be attached to the part.  A home plate design is one way to reduce the amount of paste.

Tombstoning is characterized by discrete parts “standing up” on end.  There are numerous causes for tombstoning, the most common due to poor wetting on the opposite end of the component.  In addition, an inert atmosphere can actually introduce tombstoning into your reflow process, due to cooling rates.  Large Δ T’s in your reflow process may cause tombstoning right before and after reflow.  Component and/or paste mis-registration may also cause tombstoning.

Share

Characterize your Thermocouples

characterizetcsTCs are sensitive instruments of measurement. TCs are abused mainly because they are misunderstood. First, go out and purchase a set of TCs with the proper rating for your process with the least amount of variability.  Look at the thermocouples that you are using.  Organize the TCs according to observed temperature values by plugging them into your profiler. For now, we are not going to enter into a discussion about electromotive force, thermometric sensitivity, linear vs. non-linear, TC conductor length, weld type, or even conductor material. What we are looking for here is that we are dealing with Type “K” thermocouples.

To characterize the thermocouples, you will need a few items.  First, get a new set of thermocouples, where the welded bead has been pre-formed by the same procedures. The length of the conductors (the length of the TC) should be more or less the same for each TC, with the method of TC attachment remaining constant.  My preferred method is Aluminum Tape (see TC attachment in Chapter 7 – Thermocouple Attachment Methodology and Materials).

Here are some suggested materials to use for characterization:

1. Three (or more) sets of Thermocouples  (Type “K”, all from the same manufacturer).

2. One stainless steel plate (if the same plate is used for each run, it can be made of any material as long as it is capable of withstanding the designated highest temperature for extended periods of time).  I recommend 1 or 2 mm. stainless steel, 12 inches long and 8 inches wide.

3. Squares of Aluminum tape to attach the TCs to the plate. Squares should be of equal size, with method of attachment consistent from TC to TC.  Remember, we are looking for reproducibility.

Run the plate and profiler through the reflow oven. Be certain to document as much data as possible for the sake of the experiment and for reproducibility, especially with respect to oven set points, conveyor speed, TC attachment, distance of the profiler from the plate, length of the TCs, internal profiler temperature, and any other data point that varies, having an effect on the reflow process in the future.

Repeat the run several times. Like most data, the bigger the sample, the better the results. Take your time to set up this effort.  If you are not getting similar results each time, then there is something wrong with the set-up of your experiment.  Go back and check the repeatability of each step.

Here is an example of a run where I dumped my TC readings into a spreadsheet.

characterizationtcexcel

I had over 300 samples for seven TCs.  The average TC reading for each TC is shown in the last row.  The spread from the lowest value of 149.5°C to the highest value of 150.6 °C, shows a variation of 1.1°C, which is within the +/- 1.2°C  rating for type K thermocouples.  Since I went to all this effort, I decided to pick from my study: TC#2, #4 and #7 for profiling my next PCB, knowing that my variability is no longer 1.1°C, but now .3°C, which is the difference of 150°C and 149.7°C.

The point is this, it may seem like you are splitting hairs, but 1°C here and 2°C there has a cumulative impact.  If you had a process that only allowed for, perhaps, a 5°C process window, which is not unusual today and you could cut out half of your variability due to your facility, equipment and TCs, wouldn’t it be worth it?

Share

Thermocouple Attachment

There are several methods to attach thermocouples to PCBs, some better than others. When attaching TCs, they should be strategically attached to areas that are dissimilar in terms of mass, location and known trouble spots. They should also be isolated from air currents. Lastly, the placement of several TCs should range from populated to less populated areas of the PCB for the best sampling conditions.

Several methods of attachment have been used over the years. These include: epoxy, high temperature solder, Kapton®  and aluminum tape. I have observed all four methods in the field with various levels of success for each method.

Epoxy

I find that epoxy is very good at securing TC conductors to the profile board to keep them from becoming entangled in the oven during profiling.  Epoxies come in both insulator and conductor formulations, so you need to check the specs, otherwise an insulator can play a negative role in the collection of profile data.  The ability to apply this adhesive in similar quantities and thickness is very difficult and even harder to measure in quantitative terms. This, of course, decreases reproducibility.

If you insist on epoxy, it is always wise to check the website of the epoxy that you are using to review the properties and the specifications of the epoxy.  Epoxy will function within a wide range of temperature tolerances.

High Temperature Solder

The properties of solder used for TC attachment is quite different from that of electrically connective solder. Of obvious consideration, is the melting point of the attachment solder, which must be higher than the melting point of the reflow solder paste.

Also, keep in mind that the flux used for attaching high temperature solder must not interfere with your reflow process. This is especially important if the profile board is used as a finished product!

High temperature solder is not the best choice to use for TC attachment for a couple of reasons. First, it has the same pitfalls as epoxy, in that the quantity of solder needed to adhere the TC to a substrate varies from location to location.  Secondly, solder is conductive; and therefore, it has been known to short- circuit thermocouples.  Keep in mind that a thermocouple works when two dissimilar metals are apart from one another and only joined at the bead.  Take a minute to look at the welded end of a thermocouple.

Generally, there is a short length of conductor that is exposed to the temperature gradient. Together, this exposed area, along with the physical weld produce an EMF(Electromotive Force). It is essential that the conductors and the weld are in a homogenous environment within the temperature gradient. It would be very difficult to simulate this environment with solder for each thermocouple on a PCB since several thermocouples would be required. The same is true for epoxy.

Kapton®

Kapton® tape is one of the most widely used tapes and methods for TC and TC conductor attachment. There are several sources for Kapton® and most properties are the same. Some advantages are: ease of use, cost and non-permanence.

Although most tapes are the basically the same, similar tapes and specifications can be different enough to affect attachment. Also, when several layers are applied, each layer will have an additive effect on the insulation and can negatively impact your profile. It is best to apply a thin layer. Another important disadvantage of this tape is that the PCB has to be very clean and smooth to achieve an airtight cover over the thermocouple weld and conductors. For this reason, it is not my first choice for TC attachment.

Aluminum Tape

There are several sources of aluminum tape with various thicknesses and density. I prefer to have a less dense and thinner aluminum tape to minimize the effects of the tape directly on the TC weld. Heavier aluminum tape can defuse the heat transfer through the tape and act as an insulator. Keep in mind that you are looking for the method of TC attachment that has the least effect thermally but with maximum ability to adhere the TC to the contact point of data collection.

Low density aluminum tape allows for heat transfer to the EMF-producing area of the TC. The thermal conductivity of the aluminum tape allows for even conduction when the thickness of the tape is fairly consistent in the EMF-producing area of the thermocouple. Additionally, aluminum tape is not permanent, which is good because the board you are profiling can actually be sold, but you might need to reapply tape to the same profiling board after a couple of profiles.

Aluminum Tape Used Along with Kapton®

Now for a dynamite combination: Kapton® and Low Density Aluminum tape. Used together, they produce the least invasive methods of attachment.  Using Kapton® tape to hold the aluminum tape in place and secure the thermocouple conductors is an effective use for Kapton® tape. A common method of TC attachment is called Window Paning (see figure 7-6).  Use Kapton® around the aluminum tape as an anchor, while keeping only the area of attachment in contact with the aluminum tape. This gives your thermocoupled board many more uses before it needs to be re-taped.

Share

TC Prep

TC Prep

TCs are two dissimilar metals joined by a welded bead. In order for a TC to read the temperature at any given point, the welded bead must come in direct contact with whatever you are measuring. The two dissimilar wires must remain separated and only joined at the bead; otherwise, your reading is no longer at the welded bead but wherever these two metals first make contact, rendering the reading invalid.

I have seen plenty of twisted TCs and TCs cut to length!

twistedscissors

You will not get an accurate reading if your TC loses contact with the attachment material. The TC welded bead has to remain in direct contact with your component. A zigzagging TC reading on your profile graph is a sign of a loosely attached TC.  This is why I am not a fan of TC probes since they can jump and bounce on the board.  Twisting your TC, will also give you a false reading, since your TC will measure from the first twist or point of contact.  I’ve seen plenty of TCs sticking out of solder or epoxy where this point of first contact is an inch off the PCB. In this case, you are measuring air!

Take the time-every time– to prep your TCs so that they remain apart and a welded bead remains in direct contact with your component. I use a hand pick and apply upward pressure toward the bead to separate the two wires. It will not break since it is a welded bead and should be solidly attached.

Share

Ramp Soak Spike

rss-profile

RAMP

Ramp is defined as a change in temperature over time, expressed in degrees per second.  A common value is 4°C/sec, though many component and solder paste manufacturers insist on 2°C/sec.  Who ever said reflow was easy?

Change in temperature (Δ t) with Respect to your Components

Many components have a specification where the rise in temperature should not exceed a specified temperature per second, such as in the 2°C per second example already given. The solder paste manufacturer will require comparable upper limits.  Tinned components (containing solder) and solder, in this case, behave in a similar ways.  Rapid evaporation of the flux contained in the solder paste can lead to defects, such as, but not limited too, lead lift, tombstoning and solder balls.

Possible Moisture in your Components

Rapid heat introduction to a component can lead to steam generation within the component if the moisture content is too high.  Microcracks will likely appear under these conditions.

SOAK

In the soak segment of the profile, the solder paste will approach a phase change.  The amount of energy introduced to both the component and the PCB will approach equilibrium.  In this stage, most of the flux should be evaporated out of your solder paste, but this can also depend on the type of paste.

The duration of the soak varies by solder paste manufacturer’s specs.  The mass of your PCB also factors into the required duration of the soak. Again, a balance of heat transfer and flux evaporation must be achieved. Too rapid of heat transfer can cause solder splattering and the production of solder balls, bridging and other defects. If the heat transfer is too slow, the flux concentration will remain too high and adversely impact the wetability of the bond pads, resulting in cold solder joints, voids and incomplete reflow.

PEAK

After the soak segment, the profile enters the ramp to peak segment of the profile, which is a given temperature range and duration of time that exceeds the melting temperature of the alloy, as determined by the solder paste spec.  A successful profile will typically range in temperature up to 30°C higher than liquidus, which is ~183°C for eutectic and ~ 217°C for lead-free. It is VERY important throughout the peak segment to consider the temperature tolerance of your components.

COOLING

The final area of the RSS Profile is the cooling section. A typical specification for the cool down is not to exceed -6°C/sec (falling slope).  Check the spec of your paste, but don’t forget to also check the specs of your components.  Certain packages may be sensitive to rapid cooling.

Share