A significant problem when manufacturing printed circuit boards, is the warping of the normally flat printed circuit board.
The warp can prevent pick and place machines to reliably and accurately place the components. A warp can cause the wave solder machine to pick up solder and flood the board, or result in a warped PCB which does not fit in a guide rail, housing etc.
One thing to realize is most of the time the warp is not the printed circuit board manufacturer’s fault. There are a few aspects that can cause the warp and all are known and preventable.
PERCENTAGE COPPER OF PCB LAYERS
The number one reason why a PCB will warp is uneven or unbalanced percentage copper on different layers. When a design is presented to a PCB manufacture, they will run a check of the percentage of copper on each layer. If the design is balanced where the copper plane percentages are even about the centre then all is fine. In reality however, this is seldom the case. An example would be an 8 layer PCB with copper power planes on layers 4 and 5 with signal layers on the other layers. The inner layer 4-5 has almost full copper percentage on both sides. The rest of the layers are low copper percentage signal layers. The stresses which will be locked in by lamination will even out.
When raw laminate is made by the manufacture the resin, glass fibres and copper are all pressed and heated up and cooled down at the same time. The sheets of laminate the PCB manufacturer buys are very flat. However, they do contain locked in stress. The top and bottom copper foil has a lot of strength to control the movement of the glass fibres and the resin, it is called modulus. When the sheet is laminated at high temperatures the glass fibres and the resin all move around by different amounts. The woven glass fibres are not necessarily the same density in the x and y directions, called the Warp in the X and Weft or Fill in the Y. Glass prepregs such as 2113, 2116 have almost even numbers of fibres in both directions, where prepreg such as 1080 and 7628 has a larger different number of fibres in the two different weaves which means it moves more in one direction than the other, creating warpage.
|Prepreg style||Warp Fill||Percentage difference|
|1080||23.6 x 18.5||27|
|2113||23.6 x 22||27|
|2116||23.6 x 22.8||3|
|7628||17.3 x 12.2||41|
For the lowest warpage select 2113 or 2116 as your prepreg.
When the glass fibres and the resin are laminated with the copper sheets, each part of the package has a very different CTE and modulus.
|Glass Fibres||CTE 3-6 ppm/c||Modulus 60-80 GPa|
|Resin||CTE 58 to 200(Tg)ppm/c||Modulus .7 to 4 GPa|
|Copper||CTE 17 ppm/c||Modulus 115GPa|
The copper is the stronger force of the three, the glass fibres are half as strong but only expand 1/3 the amount as the copper, so they are a controlling factor in the X-Y axes only. The resin is the weakest as far as strength goes but expands 3 to 10 times as much. Copper is strong enough (modulus) to keep the glass fibers and epoxy in check. However, once a significant part of the copper is removed by etching, locked stresses in the resin are released resulting in different rates of expansion within the PCB. The percentage of copper etched off directly affects the amount each layer expands, thus creating warp.
One of the main challenges of assembly is processing a warped board or a flat board is that it will warp further as the heat of the soldering releases the locked in stress of the PCB.
The single most significant reason for PCB warp is unequal thickness of Layers of copper not balance around the center layer of the pcb. In multilayer PCB, distributing copper area and weight equally about the theoretical centerline (the middle) minimizes warpage.
Most common PCBs are based on a laminate of multiple layers of woven glass fibre cloth and epoxy. These laminates have viscoelastic material properties. This means that the resins elasticity and thermal expansion properties drastically change above Tg. When soldering the PCB assembly, the laminate will be exposed to temperatures above Tg. There are various struggles being fought inside the PCB. The epoxy which expands the most but with the least modulus (strength) will try to expand in all directions, the glass fibres and copper layers in PCB will use their lower CTE and higher modulus to restrict the epoxy expansion in the x-y direction. Therefore, the Z axis is relatively free to expand which can result in some degree of warpage, sagging or bowing between corners.
Copper percentage ratio of an unbalanced PCB design after etching for each layer is shown below, the second row of percentages is after improved copper pouring percentages.
|PCB design as supplied||PCB design after pouring to equalise|
|Layer 1 56% partial ground plane||56%|
|Layer 2 24% signal plane sum of layers 1-4 = 118||70% sum 1-4= 229|
|Layer 3 15% minor signal plane||70%|
|Layer 4 23% signal plane||33%|
|Layer 5 22% signal plane||26%|
|Layer 6 76% ground plane sum of layers 5-8 = 226||76% sum 5-8 = 230|
|Layer 7 74% power plane||74%|
|Layer 8 54% BGA plane with ground areas.||54%|
|Warpage 3 percent||Warpage .25 percent|
In the above typical example, the original PCB design as supplied would have resulted in over three percent warpage. After the designer repaired the files with a copper pour equalization the distribution of copper among all dielectric layers is even about the center. The PCB would now expect warpage to be less than 0.25%, well below the 0.5% specified.
SOLUTION FOR PRINTED CIRCUIT BOARD WARPAGE
The main solution for PCB warpage is correct copper weight and copper etched percentage balance between dielectric layers and copper layers in PCB center. For proper warpage control during the soldering and assembly process, clamp down fixtures to hold the PCB flat during soldering should be considered. Component balance should be aimed for through component layout, thermal generation distribution of heat producing components and assembly distribution.