What is PCB Warpage?Warping is equivalent to twisting or bending out of shape. Therefore, PCB warpage is an issue involving alteration of PCB shape. Several reasons can cause this unintended change in the board's geometry, but most relate to heating (thermal stress).
The Hazards of PCB WarpagePCB warpage causes incorrect component positioning in an automated surface-mount assembly line. Also, it will be impossible to insert the components into the respective holes in through-hole assembly lines. It might seem like a small problem, but this inaccurate positioning can damage the pick-and-place machines.
A pick and place machineBesides that, if warping occurs during soldering, it will be challenging to cut the components' feet neatly. The bent shape and component feet will make it impossible to insert the boards into your machine's socket or chassis for your project.
Typical PCB Deformation ShapesThe following are the four typical circuit board deformation shapes.
- Bowl: This deformation has a sunken middle section, resembling a bowl
- Twist: Occurs parallel to the diagonal across the PCBs surface, elevating one of the corners above the plane containing the other three
- Arch: An arch deformation is the opposite of a bowl, having a curved, elevated mid-section
- Saddle: This deformation has a sunken mid-section extending across the horizontal plane
Deformation surfacesSource: Wikimedia Commons.
What Causes Warpage PCB Issue?The modern PCB manufacturing technology ensures warpage does not exceed 0.5% in all stages of the assembly process. Therefore, if your board bends past the 0.5% allowance, it could be due to the following causes.
Board DesignUnbalanced layers or misaligned fibers in multi-layer boards can cause warping. The twist and bow deformations typically appear when the fiber layers are not perpendicular to one another. Therefore, you should distribute the weight and copper traces equally around the neutral axis (theoretical centerline) during the manufacturing process to reduce warping.
Thermal Expansion/HeatMost PCBs consist of an epoxy-glass laminate (multiple layers of epoxy and weaved fiberglass). This laminate has a viscoelastic material property that changes drastically above a specific temperature known as the glass transition temperature (Tg). Therefore, properties like thermal expansion and elasticity will change when you expose the laminate to temperatures higher than the Tg in the soldering process. The laminate will try to expand in all directions, but the through-hole components will restrict this action, creating the potential for warpage.
Component Weight, Shields, or Heat SinksComponents can also create a warped board. Heat sinks can limit expansion locally by cooling some areas faster. These components are also heavy and can press down on the unsupported PCB center or other weak sections. This pressure will result in sagging during the transition phase, resulting in permanent deformation. Additionally, shielding or mechanical devices that limit board movement can create negative changes in coplanarity due to unequal expansion. Other factors include the following:
- Conductor pattern and circuit area balancing
- The cured sheet and inner layer's weft direction
Component on pcb
Prevention of PCB Warpage and DeformationIt is vital to tackle and prevent the warpage problem from the early stages using the following methods.
Improving the Engineering DesignWhen designing the circuit board, ensure the:
- Prepreg and multi-layer core boards use the same supplier's products
- Semi-solidified sheet/ interlayer prepreg arrangement is symmetrical
- The circuit pattern's graphic area on sides A and B of the outer surface should be as close as possible
Baking the Board Before CuttingBaking before cutting the copper-clad laminate eliminates moisture in the board. Additionally, it solidifies the board's resin and eradicates any remaining stress.
Relieve Stress after LaminationStress relief involves several steps that include hot-pressing and cold-pressing the multi-layer board. After that, mill off the burs and place the board in a flat oven at 150°C for about four hours. This last step should release stress gradually in the PCB and cure the resin.
Distinguish the Prepreg's Latitude and LongitudeAfter laminating the prepreg, the warp and weft shrinkage rates differ. Therefore, you must distinguish the warp and weft directions during blanking and lamination to prevent random stacking. If not sure about how to distinguish these, check with the supplier or manufacturer. Usually, the rolled prepreg's rolling direction is the warp direction, while the weft direction is the width direction. On the other hand, the long side of the copper foil board is the weft direction, while the short side is the warp direction.
Thin Plate Straightening During ElectroplatingIf using the ultra-thin 0.4-0.6mm multi-layer board for pattern and surface electroplating, ensure you have special clamping rollers. Clamp the thin plate on the automatic electroplating line's flybus, then use a round stick to clamp the entire flybus. After that, string the rollers together and straighten all plates to prevent deformation post-plating. If you skip this step, the sheet will warp after electroplating a thin 20-30 micron copper layer.
PCB copper layer electroplating machineSource: Wikimedia Commons.
Board Cooling after Hot Air LevelingThe high temperature of the hot air used to level a PCB will impact it. Therefore, you should place the board on a level surface after flattening for natural cooling.
Warped Board TreatmentBoard flatness checking is a necessary step during the final inspection. If there are warped boards, they should go back to an oven for baking at 150°C for 3-6 hours under heavy pressure. After that, they should undergo natural cooling at high pressure. This process should save some of the boards. Others need to go through these baking and pressing processes two to three times to level out.
Robotic system for automatic checking of printed circuit boards
Use High Tg PlatesThe higher the Tg value, the slower the material softens to a rubbery state after heating. Sheets with high Tg values can withstand stress and deformation better but are also expensive. The alternative is to use a thick board, but most modern electronics require thin PCBs to have sleek profiles. Therefore, you should balance between Tg and thickness to get the best value for money.
Reduce the Board Size and Number of PanelsMost back weld furnaces use chains to drive the circuit board. Large PCBs will have broad sections hanging between the chains, creating a higher risk of warping due to weight. Therefore, try to reduce the board size or use the long side as the edge on the chain if the board is rectangular.
Use a Tray JigIf you cannot reduce the PCB size, the alternative is to use a tray jig or reflow carrier for support during the soldering process.
Ditch the V-Cut Divider for the RouterLastly, try to switch the v-cut divider for a router because the former destroys the board's structural strength.
PCB Warpage MeasurementThe only way to measure the PCB warpage is by generating a Moire pattern distribution diagram. This diagram bases its dimensions on the following.
- Geometric interference between the sample's shadows and reference gratings
- Calculation of the relative vertical displacement for every pixel's position
Printed Circuit Board Warping And Leveling Method
Flatten Warped Plate in PCB ProcessDuring the PCB fabrication/manufacturing process, you can level a plate using a roll-type leveler, then proceed to the next step.
PCB Processing on a CNC machine