|Ask the Experts|
November 20, 2017
What Causes SMT Component Shift?
We are using two identical reflow ovens and both are properly maintained. We are also running the exact same boards through both ovens, yet we are experiencing component shift with only one oven. What is the reason and the solution for the component shift we are experiencing during reflow?
|Expert Panel Responses|
A great question that spawns other questions and hopefully an answer. Here goes: 1. First, we need to confirm that the ovens are truly "identical." The best way to do that is with a thermal profile. If the two ovens are indeed producing the exact same profile on the same test vehicle, then they can be considered identical. Many ovens can have differences from one to the next that can contribute to performance differences. Some examples: a. The thermal controller in the oven can have tolerances of up to +/- 1 deg C b. Thermocouple position within the tunnel - the height of the oven's control t/c in relation to the product can affect the results at board level c. Thermocouples have a tolerance of up to +/- 1.8 deg - so the control t/c can have such a range AND the t/c you are using on your test vehicle is subject to the same tolerance. The tolerances can add up so a range of as much as 4-5 deg C can be seen simply due to tolerance stack up. There are matched sets of t/c's available that can mitigate some of the stack up and typically oven controllers can be calibrated or offset to create a match between ovens so some of this can be "tuned out." Bottom line is to ensure the profiles are truly the same first. Next come some questions: Does the SAME part shift ALL the time? Is the shifting at random locations and random times? Let's look at both: A) The same part shifts all the time: We need to confirm that the placement of paste and component is accurate and identical. My guess is that these two ovens are being fed by two separate printers and Pick and Place systems. In some cases, if the paste is slightly mis-registered or if the component is slightly mis-placed on one line the component can shift or tombstone due to an imbalance of wetting from one termination to the other or due to a lack of tack on one side. The part can be perfectly placed and pass the vision check on the P&P machine but be high the Z direction. This results in movement later in the oven. So sometimes a slight printer adjustment or Z-Axis adjustment in the P&P is all it takes. In rare cases, the air flow in the oven can be the culprit. If a small part or MELF is shifting, high air velocity or local jets can cause movement. In some ovens the air is transferred through very small pin-sized diameter holes in the heater module to the product. These small jets can create locally high air flow which can move parts under the right circumstances. It is rare but an easy way to confirm this is to reduce the fan speed in the oven and see if the problem goes away. If the oven doesn't have fan speed control, you can block the holes directly above the component in question using Kapton tape for a very primitive yet quick check. If there is a local current or eddy, a simple shift of the conveyor rails one way or the other can usually do the trick. That is not to say that another problem might not be created somewhere else so fan speed control may also be desirable in terms of process control and repeatability B) Different parts shift or the shifting is random: Typically the main reason for components to move is a mechanical one. In other words, a force is acting on the part. There are a few forces that can act on a component: 1) During the heating process the solvents in the flux boil off and if the heating ramp rate is too high the small bubbles pop and can physically move small parts. So profile can have an impact - but as noted above we have already ruled out profile - I just wanted to note that here for folks who might see movement in a single line environment. 2) Handling - big cause for component movement is handling. Not so much operator handling but automation. The hand-off between the upstream conveyor and the oven can be a bumpy road. If the two conveyors are not properly aligned, then your perfect print and perfect placement can be for naught if the board catches an edge from time to time as it goes into the oven. Or sometimes the conveyor can put the board on top of a chain link (instead of on the pin) in the Oven conveyor chain. As the board rides through the oven, it can fall back down onto the pin and this movement can be just the mechanical force to cause component movement. 3) Oven conveyor - if the oven's rail system is not set up properly the board can be pinched in the oven tunnel. The mechanical forces associated with pinching, bending and sometimes popping up the board can easily move parts. Areas to check: a) Rail set up - sometimes operators will set up the conveyor rails too tightly. The board expands as it gets hotter and the rails need to accommodate this expansion. We typically recommend 1.5 - 2mm of clearance between the edge of the PCB and the chain link to allow for expansion. In many cases, operators will make up a feeler gauge that is 1.5mm thick (or use a piece of .062 FR-4) that can be slid in between the PCB and chain link for easy set up. b) Rail warpage or parallelism - if the rails are warped inward or if they are not parallel they can also pinch the PCB as it expands in the heated tunnel. The problem with pinching is that the board may simply be pinched and bent for a short time or pop up and come back to rest on the conveyor chain so when it leaves the oven it shows no evidence of the manhandling it has endured. An easy way to test for this is to set up the oven conveyor with the rails 0 - .5 mm larger than the PCB width. With the oven open (and cold) slide the pcb through the oven by hand and see if there is any binding or rough spots along the road. Hope this is helpful. If there are any questions or if we can assist please feel free to contact me or any of the Heller team and we will be happy to assist.
Heller Industries Inc.
I would start by saying that something is different between the two process lines and you need to find out what it is. Many times the reflow oven is blamed for a problem because the problem is first seen there but the root cause can easily be somewhere else. In this case we do not know the magnitude of the problem or details about which components are moving, but there a few things you can look at. The first is to make sure the reflow oven entrance curtains are the same length and do not touch the component. Next, confirm that all the recipe parameters are the same between the ovens, including the static pressure (if controlled). Then profile the two ovens to make sure the ramp rates etc are the same. Next ensure that all the belts on the problem line are running smoothly and nothing is contacting the erratic component. I said all belts because there is more than one piece of equipment that can cause the problem. Then look at the screens and screen printer to ensure that they are putting down the same amount of paste, especially at the location of the components that are shifting. Are the two pick and place machines putting the components in the same place? If observing the above does not answer the question, you will need to do some crossover of boards between the two lines to isolate the problem. If the magnitude of the problem is large this will be easy and it can be done with a few boards, but if the occurrence is low it will be hard or even impossible to do. The simplest place to swap is just before the reflow oven, but it can also be done after the screen printer, etc. There is no simple answer. It takes detective work, but at least you have two lines to compare so all you need to do is find out what is different on the problem line.
Manager, Process Technology
Not all reflow ovens are created equal. Also, if you take perhaps the best most controlled oven on the market of the same model and configuration, two seemingly identical ovens can yield different results. How can this be? You spent good money on gear you expect to perform! There are both internal and external factors that have influence on a reflow oven's ability to create your profile that have little bearing on the make and model you invested in. Let me give you some examples. Some ovens can create their own internal isolated environment, but many are still influenced by how much CFM your exhaust stacks are pulling. If one stack has more flux build up than the other this will cause each oven to have slightly different thermal behavior. Many oven controllers can have a +/- 5 degree C variance. If you think about it, in an extreme example one oven could be at -5 C and the other at +5 C, with a net difference of 10 C! Your oven may be set at and reporting a certain belt speed. Both of your ovens are set to the same conveyor speed, but when you go to verify this speed, it is not uncommon to find differences. Small differences in belt speed have a huge impact on a profile. Remember when you do run a profile, you are only taking a snap shot. Let's go back to your conveyor speed as an example. Let's say your conveyor sticks, but only at times. Periodic profiling is catching it when your conveyor is running as it should or when it was sticking? Seems silly each of these in isolation, but when you add them all up, now you can see how even the best maintained reflow ovens of the highest possible workmanship can behave differently. At KIC, many of our customers utilize 24-7 continuous monitoring, where each and every PCB that passes down the line, a profile is created by the KIC 24-7. These customers may have dozens of lines, every aspect identical in except one, you guessed it, each line creates a slightly different profile. There is proof right in front of your eyes, since you can see in real-time how there is variability on a single line throughout the day never mind variability from line to line. When all this variability can be controlled, I need to look for a new job....
Global Account Manager
There has to be some differences in theoperation of the similar equipment. I would check for the following:
Vice President, Technical Director
The question does not specify if all components are shifting or a particular subset. Components with a high mass to lead area ratio are more vulnerable to acceleration. Check for component shift caused by acceleration and deceleration on the pre-oven conveyors and any abrupt force applied to the board due to conveyor misalignment or belt speed differential when the boards are passed to the oven conveyor. Those forces are more likely to cause component shift than anything inside the oven aside from a low hanging curtain. If acceleration and deceleration can be eliminated as root causes, more information on component type(s) affected would narrow the list of possible root causes.
Application Engineering Supervisor
Two identical reflow ovens that are properly maintained does not guarantee that the ovens are performing identically. Have you run profiles on the assembly through the two ovens? I suspect that you will see a differences in the ovens such as ramp rate, peak temperature and time above liquidus. Other factors that will affect the thermal performance are the age, make and condition of the ovens, efficiency of the circulation fans and heating elements. Run profiles on the assembly through the two ovens. Use the profiler software to overlay the two profiles one on top of the other. Then you can really compare the performance and find the problem.
National Sales Manager
Component shift on identical boards through same type of oven can be due to air flow through the nozzles of the oven especially if it is small components. Can you adjust the air flow and reduce to the good ovens rate of flow?
Technical Sales Manager
BLT Circuit Services Ltd
Assuming that there is one placement line feeding two ovens, I would expect that the "problem" oven is either off level or one of the heater banks or blower banks are out of calibration. Of course, if they are 2 independent lines, then it could be the pasting or placing operation causing the component shift.
Regional Sales Manager
OK International Inc.
There are more questions to bring up in orderto get to the bottom of this issue:
Engineering and Operations Management
I have seen this a number of times through the years and it has almost always been caused by either bad bearings, worn idlers or the chain is seized and popping as it goes over the idlers.Greg Voss
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