|Ask the Experts|
June 23, 2021
Circuit Board Bow and Twist
Board flexing has recently started to occur on one of our circuit board assemblies. This assembly has a 40 mm PoP BGA component. The BGA has an integrated heat sink.
We are using a 3 zone reflow oven. The profile worked without issues for at least 6 months. The problem with the boards bowing, or curving upwards towards the middle of the board, has just started recently.
Yet our process has remained the same and there have been no changes. Any help is appreciated.
|Expert Panel Responses|
There are factors both in the PCB manufacturing process as well as the assembly process that can have affects on board flatness. Some of these items can be variable (not consistent) depending on the control of a various manufacturing or assembly process.
On the manufacturing side: warpage can be introduced by changes in the material being used, the stack up of the board being changed or variations in process. Most of the process variations that could be responsible are heat related lack of control of temperature (greater fluctuations) during lamination, thermal curing of soldermask, legend, HAL process, or baking for flatness at the finished board level.
These fluctuations in temperature can typically add a stress or strain to the material that can be released when it is heated (during assembly) during subsequent processing.
Assemblers will, on occasion, bake boards prior to assembly to fix other quality problems like out gassing etc. These baking processes can also introduce strain and stress into a PCB. Another possible source is boards that are inadequately supported during a wave soldering step (more of an issue on larger boards).
The final area that can introduce warpage into a PCB is the design itself. If there is an unbalance of copper through the design (layer to layer) or unbalanced dielectric thicknesses throughout the design, these can add stress and strain to a board as well.
Trying to distribute plane layers and signal layers evenly through the build, using consistent dielectric thicknesses (at least symmetrical top to bottom) and trying to have well distributed copper on each layer during layout will help make the design less susceptible to warpage.
Director of Marketing
A three-zone oven? What? That seems crazy in terms of being able to control a repeatable reflow process. If the board is bending, talk to your PCB manufacturer. They could be doing a second pressing operation, which just hides warping issues until you go through reflow.
CEO & Managing Partner
Your problem with excessive bow and twist may be addressed by the direction of the warp and weft, on each dielectric layer of the glass fibers in the PWB. The warp is the direction of glass that runs length wise, under tension, during the weaving process.
The weft is the direction in which the glass fibers go around the warp fibers during weaving. Typically the warp is under more tension and the weft is under less tension. This accounts for the difference in coefficient of thermal expansion (CTE) in the x and y direction in FR4 material (14 and 17ppm/C).
What can happen is the fiber glass may be laid up with the warp of every layer in the same direction; this can make the board more prone to bow and twisting. If the fiber glass is laid up in a manor that every layer has the warp laid down at right angles to the layer above and below it then bow and twist may be reduced. You can specify the direction of the warp and weft, or have the warp and weft laid up and right angles with each dielectric layer,with the better fabricators.
PWB Interconnect Solutions
There are four main possible causes to look at:
It is possible that a combination of design issues and fabrication process variation is leading to higher warpage on some lots of boards. You can check this by reviewing the manufacturing date code on the boards that are warping/not warping, and determining if it is correlated to a specific date code.
Also, look at the oven conveyor, and make sure that if it is an edge conveyor that the width is not set too tight. Even if it does not appear so at the entrance, it can be too tight internally due to non-parallel rails and due to growth of the board during heating. If the oven has a center support, make sure that the support is properly positioned and is not causing the center of the board to be elevated.
Assuming the reflow process and zone temperatures have not changed (which you should verify with thermocouples attached to various sections of the board), you must assume the PCB supplier made a change to one or more layer materials, most likely the pre-preg.
Another possibility is that the laminate layer supplier made a change and did not inform the PCB fabricator. My suggestion would be to first do a thorough calibration verification of the reflow oven temperature and speed, and once that failure mode is ruled out, conduct a detailed spec review with the PCB supplier.
No mention was made about the PCB dimensions (including nominal thickness and tolerances), but if the problem persists, there are a number of ways to maintain planarity of the board including stiffeners, and/or different PCB stack up materials and methods.
Round Rock Consulting
Given that the circuit pattern and assembly processing are unchanged, bare board processing can be considered. If the manufacturer fails to consistently orient the grain direction of the laminate, variations in flatness may be realized later-on in their processing.
If so, one ill-advised "fix" might be attempted: To flatten the product under uniform weight, in ovens of a temperature sufficient to soften and relax the laminate. After adequate time-at-temp the oven is then turned OFF, the doors opened and the product allowed to cool, still underweight.
But the cause of the stress remains, now restrained by laminate that is cooled, firm and flat at room temperature. However, when the PCBs next experience high temperatures (such as those produced by your Reflow) the laminate may soften and, unrestrained by a uniform weight, yield once again to the effects of internal stress.
Circuit Connect, Inc.
Instability of the inner layers or improper layering of the board during manufacturing may be contributors to this issue. IIT manufactures board stiffeners from titanium to remedy issues like this.
They slide onto the edges of the board easily and keep the board parallel during reflow. If the inner layer instability is severe enough, warping will still be a possibility once the stiffeners are removed.
Integrated Ideas & Technologies, Inc.
There are several potential reasons for bow and twist of finished PCB's some of which have been mentioned.Michael J Gay, Isola Laminate Systems, USA
Copper CTE 17 ppm versus a range of 11-24 ppm x-y CTE for laminate. Low resin content dielectrics have low x-y CTE values and high resin content dielectrics have high x-y CTE. This can lead to the following problems:
All of the experts here had excellent and very experienced inputs. Here is one more: I recently had a PWB begin to show warping issues, even though nothing had changed in the process. We double-checked everything in the assembly process and then we went to the fabricator and double checked everything there. Nothing had changed, artwork was the same, copper was from the same vendor, laminate was from the same vendor, etc.
We couldn't find anything that had changed. But the fabricator did offer up one little tidbit of info; he had changed the layout of the laminate "flat", in order to get more individual PWBs out of each FR-4 "flat" and reduce scrap. So the original 12 of the 16 boards produced from each flat had been rotated 90 degrees to create more room for 4 additional PWBs along one side of the flat, and those were positioned 90 degrees from the rest.
That was all it took to give us 12 warped PWBs out of every 16, the direction of the weave. When the fabricator went back to the original 3 x 4 array, the problem immediately disappeared. Usually, however, it is never a single issue causing the warpage. It is typically some perfect storm of two or more of the factors mentioned from both the fabrication AND the assembly processes. If it ain't broke, keep the heck away from it!
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