Printed circuits or PCBs are getting thinner and smaller as they get more complex (prevent PCB bent). But, occasionally, we run into the issue of a bent PCB or a warped board, even if a minor curve might also impair the performance of a PCBA.
The “Performance and Specification Standard for Rigid PCBs” states that the SMT boards’ maximum permitted warping and bending is 0.75%. Nowadays, the warping tolerance range for double-layered or multi-layered electronic assembly facilities with a 1.6 mm thickness is typically 0.70-0.75%. A lot of BGA and SMT boards need 0.5%. Several electronics firms are pushing for a 0.3% warping norm.
Bent PCB Test Method
Placing the PCB on a validated platform and inserting a test pin into the area of the board that exhibits the greatest warping will allow you to determine the degree of warping. It divides the test pin’s diameter by the height of the bent edge of the board.
Cause Of Bent PCB or Deformation
PCB deformation requires visuals of the substance, distribution, structure, and processing processes from a number of angles. As in the research, this article’s deformation may occur for a number of causes, and many ways to improve are considered and explained.
Because the surface area of copper on the PCB is not homogeneous, the plate will distort and flex.
PCB also has the ability to heat up and cool down. But when these vast copper foil areas cannot be distributed evenly over the same board, it will result in uneven cooling and heat speed. PCBs also have the ability to experience different types of deformation and stress depending on the board’s temperature at the time. If this temperature is too high, the board may become distorted.
The link marks on every layer of PCB (vias, TH) will restrict the board’s shrinkage.
Today, the majority of PCBs are laminated, and the connection points between layers are like rivets. Connecting points are further divided into holes, buried holes, and blind holes. Connecting points limit the effect of cold on the board and can also implicitly result in warping and plate bending.
There are numerous reasons that cause the issue of PCB bending. The most frequent one occurs when the tension placed on the PCB exceeds the stress that the PCB can sustain; the Board bending problem manifests itself right away.
The “high temperature” is the main source of stress when PCBs are processed via the reflow hot oven for Surface-Mounting Technique. The PCBs will become softer at high temperatures due to the CTE factor, as well as their feature of thermal contraction and expansion. If somehow the temperature exceeds the PCBs’ maximum TG value, the Circuit boards will bend.
Rarely do PCB designers take the edge of the PCB copper into account. The Circuit board will readily flex in high-temperature conditions if the Circuit edge does not have copper.
Also, if the SMD electronic components are hefty and the PCB size is rather large. The weight will create the Circuit board bending problem.
The PCB’s structure will be destroyed by the V-cut used to neatly separate the boards, and this will quickly lead to the Board bending problem.
How To Prevent PCB From Bending? Avoid Bent PCB
Based on over 18 years of manufacturing and installation expertise with PCBs, we’ve compiled some lessons learned and suggestions on how to avoid PCB bending that we’d like to share with you.
1. Use High TG Material
The average temperature (TG) is above 130 degrees, the average high temperature (TG) is above 170 degrees, or the average medium temperature (TG) is above 150 degrees.
Increasing the TG improves the PCB characteristics of the material. This includes humidity resistance, heat resistance, stability, and chemical resistance. There are additional high TG uses, particularly in the process of lead-free. This is because the plate’s thermal resistance improves with increasing TG value. The greater TG value guarantees that the Circuit is somewhat resistant to deformation and changes caused by the outside environment, which can prevent the PCB from bending or warping.
2. Board Thickness
Boards that are thicker will be more rigid and simpler to keep straight. Reflow soldering needs to install several components on PCB. The thick base is difficult to distort when the load and temperature change. Maintaining the thickness of the board at 1.6mm can greatly lessen the likelihood that the board will deform.
3. Evenly distributed Copper
PCB assembly finishes in a panel format. If the copper is distributed throughout the board side and connected areas after it has been panelized (positions that will be removed after assembly), the total PCB panel must be maintained. When the copper’s density and thickness are identical, warping the plate after assembly is difficult.
4. Panelize Way
It is important to keep the panel’s size in mind when designing it, as well as to keep its shape consistent and stable. A panel that is too lengthy will break in the center.
5. Reduce Reflow Temperature
If an unequal or excessive strain applies to the PCB, it will deform. From where does the board’s tension come? In the soldering reflow process, the biggest stress arises from temperature. Together with physical reasons that induce thermal expansion in all materials, the temperature does not soften but also bends PCB. As a result, it is thought to be the primary reason for PCB warping.
6. Modify Design
There will be a lot of vias, blind buried, and blind holes on multilayer PCBs. These soldered nodes encourage the development of circuit warpage and restrict the alterations that the PCB experiences when subjected to high temperatures and pressure. As a result, some layout engineers will label some spaces in the substrate’s absence of copper wiring in order to aid in the PCB’s heat dissipation.
7. Reflow Carrier
The reflow soldering carrier can decrease the PCB warping and bending. This is because it has the ability to hold the PCB despite the fact that heat will extend it. The PCB regains its warmth and hardness after the reflow process of soldering is finished. This procedure is restricted to the carrier of reflow and won’t take place in all cases. We also can create fixtures to aid in fixing, although doing so entails manually replacing pallets.
8. Do Router Not By V-Cut
If we create the panel using a V-cut, the PCB’s general structure will deteriorate, making the panel bending and warping more likely. So, we may utilize routing rather than V-cut if it’s permitted in production.
9. Warp & Wept Direction
The rate of prepreg shrinkage in the weft and warp directions really isn’t equal after laminating. While trimming and laminating, you must differentiate between the weft and warp directions. If not, the completed board could deform after lamination, which would be challenging to fix even with pressure.
The prepregs’ longitude and latitude are sometimes not differentiated and are stacked arbitrarily during lamination. This results in the multilayer Circuit board bending for a variety of reasons. How are the weft and warp distinguished? The prepreg is folded in the direction of weft for breadth and the direction of warp for thickness.
In the case of copper foil, the warp is on the short end, and the zonal is on the long end. If you’re unsure, you can ask the supplier or manufacturer.
10. Semi-Solidified Symmetrical
For instance, on a six-layer PCB, the semi-solidified panels should be in layers one and two, five and six. Prepregs and multilayer cores ought to utilize supplies that come from one vendor.
On the A & B edges of the outermost layer, the space of the pattern lines ought to be as similar as feasible.
This PCB will readily deform after etching if surface A is a big surface of copper and surface B is just a few pattern lines away.
If the line areas on both sides vary too much, users can add a few individual grids over the rare sides to adjust.
11. Baking Board
Prior to trimming the copper-clad panel, dry the panel to eliminate any moisture that might have crept in during prolonged storage (150 degrees Celsius, 8 x 2 hours).
Also, it is important to thoroughly dry the epoxy in the panel, which will further reduce any leftover tension and aid in keeping the board from warping.
Currently, numerous double and multi-layered PCBs still conform to the phase of baking prior to or after blanking. Nevertheless, certain plate manufacturers are an exception. The time necessities for drying panels in different PCB factories currently range from 4 to 10 hours. We propose selecting depending on the quality of the PCB generated and the needs of the customer for the warpage. These approaches are possible after cutting the block into panels, baking, or chopping the entire block. After cutting and baking, dry the circuit boards. After cutting and baking, dry the circuit boards.
