|Ask the Experts|
November 2, 2020
A Generic Reflow Profile
We have a wide mix of PCB assembly types. Some folks here are proposing that we use the same reflow profile setting regardless of the PCB thickness, thermal mass or component mix?
It has always been my understanding that different PCB types should use different profile settings. Is there any justification for using one generic reflow profile?
|Expert Panel Responses|
Using a generic profile in production will bring on more grief and scrap boards than you are willing to produce.
Each board assembly has a unique profile because of the variations not only in board thickness, but also the thermal mass and distribution of the attached components.
To ensure proper solder joints and successful assemblies, you need a profile for each assembly you intend to run.
Manager of Assembly Technology
In general, no there is not. Through careful testing, you may find that you can narrow the number of required profiles to just a few, however(the number depends on your product mix). The emphasis here is on the testing.Every board has unique thermal requirements.
The thickness of the PWB, amount and location of copper in the PWB, the component locations and masses, presence of "back-to-back" components on the assembly, and other variables all contribute to the thermal requirements. And don't forget that most ovens respond to load slowly enough that the second board may see slightly different temperatures than the first, and that temperatures very near the rails are almost always cooler.
There is no substitute for really understanding the thermal requirements of each assembly part number. Once you do, it may well be possible to group them according to common profiles.
Further, if you have a basic profile, and you need a slightly warmer or cooler profile, it may be possible to achieve this by small changes in the conveyor speed, as long as the timing of preheat,time over liquidus, and cool-down are not affected too much. The advantage of doing this is that profile changes become nearly instant, since no temperature settings are changed.
The only way that a single profile could be used for all board designs would be if the rate of rise and cool down temperatures are not in violation of the chip suppliers and that the time above Liquidus does not become excessive with a resultant heavy IMC being formed that would have a negative impact on reliability.
In addition, you would have to verify that voiding would not become an issue with a single profile. In other words you typically cannot use a single reflow profile for significantly different boards, i.e. thickness, copper weight, density etc.
S T and S Testing and Analysis
You are correct in your thinking that different board types, thicknesses and component densities may demand different oven recipe settings (profile). Whether a different assembly needs new oven recipe settings or not can only be shown by performing a thermal profile of that new assembly. This will allow you to see for sure of the current oven recipe settings meet the thermal requirements of the components and solder paste.
Given the thermal profile of the new assembly, you can determine if the oven recipe settings need to be changed to prevent component damage and or meet the thermal profile needs of the solder paste. Without the thermal profile of the new assembly, you will only be guessing and you will have no proof the recipe settings are sufficient.
Senior Project Engineer
Electronic Controls Design Inc
Each and every board should have its own profile. Granted there may be some boards so similar that a new reflow profile is not needed;e.g. a revised board where a few discrete components are added or deleted. There have been attempts to "profile" by board weight, or grouped by type, etc.
I have never seen this work effectively and in fact have seen lots of production material sacrificed because of lack of adequate profiling. Take the needed time and materials to develop an adequate profile.
The following is only a partial list of things that have the potential to alter reflow profile of two boards that "look" alike:
Don't think only in terms of inadequate reflow/cold solder joints. Each part used on the assembly comes with a thermal specification; generally a JEDEC specified maximum temperature. Exceed that temperature and it may either ruin the device or lessen its reliability. Do the work to ensure that the profile is suitable in all respects.
Use profile data review along with visual inspection, x-ray and even cross-sections if warranted before committing your company's money or your customer's faith in a production run. Temperature in the middle of a BGA may be significantly different than at periphery of the device. Don't guess! Measure and inspect.
How a board is instrumented with thermocouples can make a difference too. Know how to attach, where to measure and how many thermocouples are needed. There are many publications on this topic.
With today's profile tools (thermal data recorders) and profile prediction software that comes with such profile trackers, profiling is not as time consuming as it once was. Take the extra time to get it right. By the way, the same logic applies to wave soldering and automated methods of rework such as hot gas repair.
The short term justification is that it saves time (change over and recipe generation), the same way as not performing maintenance does. The reality is that unless you're using vapor phase condensation reflow, varying masses will produce different thermal profiles, and should have a unique recipe that maximizes your solders ability to solder.
This difference is related to your ovens capability / capacity to heat these varying masses in a given process time. How much difference can only be determined by profiling, and that is why we at least check the profile once on every assembly.
If your products have similar masses,similar board thickness, and similar foot prints, you can expect a similar profile will be achieved (although you have no proof unless profiled). If there is a change in any of these parameters, there will be differences in the profile, and the only way to prove that those differences won't violate any reflow specification is to profile it. If you prove one recipe will work by profiling all your assembly's, then you can use that recipe with confidence.
Can you get away with using the same recipe on every board, YES for awhile, until you get a board with enough difference to cause soldering defects.
M.O.L.E. Line Product Manager
Electronic Controls Design, Inc. (ECD)
No there is no justification for using one profile unless you really want to produce potential scrap PCB's.
I usually recommend three profiles, TO START with and then fine tune by profiling the actual PCB, populated ideally. If not populated then add 10C to each zone to allow for components. So set up a small, medium and Large profiles but use them only as a starting point.
You may in the future get confident that you will know which Circuit will fall under which profile in the future so you don't have to profile each and every board that comes through your factory. BUT until that time get profiling would be my advice.
Technical Sales Manager
BLT Circuit Services Ltd
Being able to use a single reflow oven recipe for all products would be nice but it will certainly get you in trouble if your boards are significantly different. In some cases it is possible to group boards into categories that share the same recipe only if the boards are similar.
In 2009 BTU did a study that described what happens to the thermal profile of a board when various reflow parameters are changed. One of those changes was using the same oven setting with boards with different mass. The difference in peak temperature of over 5 degrees C was seen when the PCB weight changed from 100 grams to 250 gram PCB without changing the oven recipe.
There are two papers that can be downloaded from the Knowledge Center on the BTU web site under the "About US" tab that will explain why the thermal profiles cannot be generic.
One is "Thermal Profiles - Why Getting Them Right is Important." and the other is "Oven Adjustment Effects on a SolderReflow Profile."
Manager, Process Technology
Based on the board diversity detailed in the question, it is unlikely the submitter can use just one profile for all their boards. The only justification for single profile reflowing would be a product mix that lends itself to that choice. Depending upon the heating capabilities of an oven, one may be able to use a few "generic"profiles as opposed to many PCB specific ones.
We recommend that a customer assesses their product mix, looking for groupings of PCBs that lend themselves to use of a single profile for the group. Most of our customers that use this method end up with two to four profiles that serve the vast majority of their products and a handful of specialized profiles for their more challenging PCBs.
Application Engineering Supervisor
I would definitely not recommend a generic profile. There might be "families" of boards that can use the same settings for reflow but with so many variables involved, a generic profile will not help.
The mass of the board, surface, copper plane thickness and especially the population makes a huge difference on the results. We are running a high mix, low to medium volume product and every single assembly has its own reflow profile.
Engineering and Operations Management
A generic "Reflow Profile" is a great idea but the problem is hitting it with different board designs. There is a lot you can do with a quick visual of the board and parts. What I did was to generate a small array of "Generic Profiles. For example: Light Double Sided, Heavy Double Sided, Light Layer, Heavy Multi-layer.Bob Kondner, Index Designs, USA
I suspect you are doing this to save time on the various incoming jobs. Spend a little time with qualifying a couple of test case boards and make sure you run on the hot side. Going a little low in temp is typically more of a problem than a little hot.
If you plan to do this with lead free it is more difficult. I would seriously suggest a vapor phase oven. Maybe not for all boards but just for the board of the edges of your board type definitions.
Years ago the reflow oven couldn't achieve a good delta between boards with different mass, fortunately now the reflow ovens can stabilize the delta under the process window in a group of boards with similar mass and component density, so I recommend selecting a group of boards or families and run a profile in order to know the delta between boards, if the delta is under the process window, you can do a generic profile, only for a selected group of boards.Sergio Ilescas, Arris, Mexico
Heat flow simulations have been shown to be capable of predicting the response of a PCB design to a particular reflow profile to a pretty good level of accuracy, provided the reflow furnace is sufficiently well characterised, and this can allow "virtual profiling" saving time and the cost of test boards.David Whalley, Loughborough University
However there is still a need to verify the oven performance remains the same, and just checking zone temperatures with a calibrated measurement system isn't sufficient - changes in gas flow rates within the zones are also very important. So re-checking board profiles or using an "explorer" type tool on a regular basis is necessary.
Lot of type of board lot of time and lot of many. So maybe the best solution what Bob recommended: grouping of board. And what type of solder machine talk about. Infrared oven much more problem as a forced convection oven or vapor phase oven.Imre Rajcz, NIVELCO LTD
I would listen to the experts who have replied in this column, and as for "some folks there" who are proposing that you use the same reflow profile setting regardless of the PCB thickness, thermal mass or component mix, why, I would simply thank them for the suggestion and hand them a job application for your company's Rework and Scrap Processing Area!Dean Stadem, Analog Technologies Corp.
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