Introduction
Soldermask or solder resist is the protective layer of polymer coating applied over the copper traces on printed circuit boards (PCBs) to control solder spreading and prevent bridging between pads during component assembly. Openings in the soldermask selectively expose the underlying copper pads that need soldered connections. The width of these openings relative to the pad size is known as the opening shave.
This article provides a detailed overview of soldermask opening shave including its purpose, typical values, how it is designed, considerations for different pad shapes, and effects on manufacturability and soldering defects. Guidelines are provided for calculating appropriate opening shave widths based on pad geometries and solder flow needs.
Purpose of Opening Shave
The main objectives of providing additional open area around pads include:
- Exposes the surface of pad for sufficient solder wetting.
- Accommodates registration tolerances of soldermask alignment.
- Allows a channel for outflow of excess solder away from the pad.
- Improves manufacturability by reducing probability of mask openings shrinking smaller than pads.
- Lowers risks of solder bridging between neighboring pads.
Typical Opening Shave Values
Industry standard IPC-7351 specifies a minimum annular ring of 3 mils (75 μm) of open pad area extending beyond the soldermask on all sides. However, common design values are:
- Low density through-hole pads: 5 to 8 mils
- High density surface mount pads: 4 to 6 mils
- Fine pitch components: 3 to 4 mils
Higher opening shaves up to 10 mils may be used in vibration environments where soldermask separation risks are higher.
Design Factors for Opening Shave
Key considerations while designing opening shave include:
- Registration tolerance between pads and soldermask image
- Pad shape and orientation – square pads need larger shave
- Pad density – higher density needs tighter shave to avoid bridging
- Soldermask expansion space from pad for outflow
- Copper pad thickness – thicker copper allows slightly smaller shave
- Soldermask material – photoimageable masks hold registration better
- Vibration levels – shave increased at vibration prone regions
- Rework considerations – sufficient space for rework and solder cleanup
Opening Shave for Different Pad Shapes
Rectangular Pads
A symmetrical shave of 4-6 mils on all sides is typical. The long edge shave may be 1 mil higher if length exceeds 1.5 times width.
Square Pads
Require at least 4 mils additional opening on all four sides due to higher bridging risks along diagonals.
Rounded Pads
Here adjusted shave widths compensate for shorter distance along curved edges:
Rounded pad soldermask opening (Image Credit: PCB Square)
Effects of Inadequate Opening Shave
Insufficient shave exposing the pad can lead to:
- Dry solder joints or incomplete wetting if mask overlaps pad area
- Solder masking separation under thermal stresses, closing the designed openings
- Solder spread into narrow openings increasing bridging tendency
- Voids and trapped fluxes due to lack of outflow clearance
- Inability to clean undermask areas during rework
Solder Defects Related to Opening Shave
Various soldering defects can be caused or exacerbated by improper control of soldermask opening shave:
Solder Bridging
Pulling in of solder between adjacent pads when clearance space is inadequate.
Solder Balling
Fluid solder gathering into spheres instead of wetting pad surfaces when openings are mismatched.
PCB Delamination
Soldermask separation from pad edges under vibration or thermal stresses exposes more copper area.
Solder Beading
Ring-shaped solder bead formation along pad periphery when mask overlaps pad.
Solder Mask Slivers
Sliver-like leftovers of soldermask inside openings interfering with wetting.
Guidelines for Calculating Opening Shave
1. Determine pad size and shape
- Measure length, width for rectangular pads
- Define diameter for rounded pads
2. Account for soldermask registration tolerance
- Typically around 4 mils (100 μm)
3. Add minimum annular ring width
- IPC-7351 recommends 3 mils (75 μm)
4. Provide expansion clearance
- 2 mils for most pads
- 4 mils for large pads > 40 mil sides
5. Round shave dimensions up to nearest 0.5 or 1 mil grid
- Simplifies manufacturing tolerances
6. Increase shave for vibration exposure
- Add 2-4 mils for vibration prone regions
7. Verify shave against IPC or manufacturer’s guidelines
- Reshape pad if needed to allocate sufficient shave
Conclusion
The soldermask opening shave is a small but vital PCB design parameter that prevents defects and rework in assembly by properly exposing pads for clean soldering while limiting bridging risks. Applying appropriate shave margins based on pad sizes, shapes and density allows high soldering yield. As PCB fabrication precision improves, opening shaves continue to shrink permitting further miniaturization.
Frequently Asked Questions (FQA)
Q1: How is the registration tolerance between pads and soldermask openings reduced?
A1: Using photosensitive soldermasks exposed with the same PCB pad image minimizes image translation errors. Laser direct imaging can further improve alignment precision.
Q2: Which pad shape typically requires largest opening shave?
A2: Square pads need relatively larger shave margins along the pad diagonals to avoid bridging compared to rectangular pads. Rounded pads allow tightest shave due to reduced meniscus formation along curved edges.
Q3: How does soldermask thickness impact the opening shave?
A3: Thicker masks impart greater stress on pads risking delamination and separation. This may necessitate slightly higher shave values. Typical mask thickness is around 3-5 mils.
Q4: When is a larger than normal opening shave warranted?
A4: In vibration prone environments, where differential expansion might gradually expose more pad area. Also, where anticipation of numerous rework cycles requires extra clearance for cleaning undermasks.
Q5: How is opening shave optimized for fine pitch components?
A5: Reducing shave close to the minimum recommended values allows tighter packing while preventing bridging between adjoining pads. This requires high precision imaging and registration process capabilities.
How to design pcb soldermask opening
This article mainly introduces the opening of pcb. Firstly, it introduces opening and bright copper in PCB design. Secondly, it introduces how to realize the tinning of PCB wiring. Finally, it explains the steps of how to set the opeining.
What is the pcb soldermask opening?
The circuit on the PCB are covered with soldermask to prevent short circuits and damage the device. The so-called solder mask opening is to remove the paint layer on the circuit, so that the circuit can be exposed to tin.
As shown in the above picture, it is the opening. PCB opening is not uncommon. The most common one is probably the memory stick. The students who have removed the computer know that the memory stick has a gold finger, as shown below:
The golden finger here is to opeing, plug and play.
There is also a very common function of opening the opening, which is to increase the thickness of the copper foil in the later period, which is convenient for excessive current, which is more common in the power board and the motor control board.
opening and bright copper in PCB design
In the design, customers often ask for opening and bright copper. Because the customer is also ignorant or we are not too clear about this process, it is very troublesome to communicate. In our design, we often encounter customers who need to add shields, partial bright copper on the board side, through-hole open-resistance soldering, copper on the back side of the IC heat sink, and scratch pad. According to the actual situation, let’s take a look at several sets of pictures to explain.
1, shield
If the customer needs to add a shield, then all we have to do is add a Soldmask with a width of at least 1mm. If you need to add a stencil, you need to confirm with the customer. At the same time as adding the Soldmask, we need to spread the network copper in the add mask area, and we must cover the Soldmask plane, otherwise the pcb substrate (FR4, etc.) will be exposed. Other non-local networks should not pass through the Soldmask. Adding a loosemask area to the pcb effect reveals a yellow copper. Solder mask coverage is provided for areas that are not added.
2, solder mask opening hole
In the design, we often hear the whole plate plug hole or partial plug hole. When adding the hole, we pay attention to the fact that the plug hole company name generally refuels the BGA, and vice versa. ). In general, a company that has a specification of more than 12 mils must use a solder mask opening.
3, IC thermal pad
Generally, a solder-proof PAD is added on the back of the IC heat-dissipation pad (adding a shoulder mask larger than the surface layer or equal to the surface of the surface pad) and a ground hole, and a copper-clad solder mask is placed on the back surface to better pass the heat of the surface layer. The hole in the hole is transmitted to the back of the copper skin to disperse better.
4,The Pad tin touched
In wave soldering, in order to solve the problem of tin bonding caused by the tight pitch of the pads, we will use the shape of the scratch pad. Note that it is necessary to add copper bumps of the same size as the solder mask while adding the solder mask.
How to realize PCB trace opening
In the circuit vias, it is necessary to drive 8 relays. When the multi-channel relays are turned on, the current is greatly increased. To ensure the actual effect, while widening the current line, it is desirable to remove the solder mask on the current – the green oil layer, and the board is made. In the future, you can add tin to the top, thicken the line, and pass more current.
The actual results are as follows:
The implementation method is as follows:
Draw this line in the layer of the top PCB layer (or bottom layer depends on the layer where the preset line is located), and then draw the line that coincides with this in the top solder (or bottom solder) layer.
How to set the circuit to open
The PCB design can be used to set the opening on the TOP/BOTTOM SOLDER layer.
TOP/BOTTOM SOLDER (top/bottom solder mask green oil layer): The top/bottom layer is coated with solder resist green oil to prevent tin on the copper foil and keep it insulated.
A solder resist green soldermask opening can be placed on the pads, vias, and non-electric traces of this layer.
- The pad will open by default in the PCB design (OVERRIDE: 0.1016mm), that is, the pad exposed copper foil, the outer expansion is 0.1016mm, and the wave soldering will be tinned. It is recommended not to make design changes to ensure solderability;
2, the via hole in the PCB design will open by default (OVERRIDE: 0.1016mm), that is, the through hole exposed copper foil, the external expansion 0.1016mm, the wave soldering will be tin. If it is designed to prevent tinning on the vias and do not expose copper, you must tick the PENTING option in the additional properties of the vias SOLDER MASK to close the vias.
- In addition, this layer can also be used for non-electrical routing, and the green soldering resistance should be opened accordingly. If it is on the copper foil trace, it is used to enhance the overcurrent capability of the trace. When soldering, it can be tinned. If it is on the non-copper foil trace, it is generally designed for marking and special character silk screen, which can save production. Character silkscreen.
