Defined Impedance Calculators
|
Archive for category: Tips & Tricks
Selecting the Right Surface Finish for Your PCB
|
Selecting the Right Surface Finish for your PCB
Surface Finishes on Printed Circuit Boards
Choosing the right Surface Finish for your PCB is as important as choosing a manufacturer to make the PCB.
Use the wrong Surface Finish and it you may have downstream issues during assembly or longer-term reliability issues.
The Surface finish is a protection against the elements, in short it is designed to protect soldering pads and contact pads from oxidation or becoming contaminated.
Without a Surface finish the exposed copper would oxidise, thus making the soldering of components difficult and any electrical contact pads such as edge connectors would have a poor electrical connection.
Since all finishes have advantages and disadvantages, it is important to select the right one based on the PCB design, storage, functionality and how they will be handled during assembly.
The different finishes we offer are Lead-free Hot Air Surface Levelling (HAL), Electroless Nickel/Immersion Gold (ENIG) and Immersion Silver (ImAg). All of these are Lead-free and can be used for RoHS designs as well as in a SnPb assembly.
It is recommended that edge connectors have an Electroplated Hard Gold (Ni/Au) surface finish.
Any Lead Free
If you don’t select a finish then your PCB’s will be produced on a panel with either a Lead-free HAL, chem. Ag or chem. NiAu. The finish will be defined by other PCB’s on the same panel.
However, if your design includes fine pitch pads this will automatically be changed to ENIG finish.
HAL Leadfree (Hot Air Surface Levelling)
The Lead-free HAL (or HASL as it is also known) finish provides the PCB with the highest level of solderability. It also provides the level of robustness with regards to multi-step assembly and storage at a reasonable price.
On the other hand, the HAL process requires the submerging of the complete PCB in liquid solder and thus is responsible for extra thermal load on the PCB.
For that reason, it isn’t the best choice if your board requires small via holes or the board exceeds a normal thickness (when the aspect ratio is high).
Another aspect of this surface finish is less flat than other surface finishes, although we try to achieve the flattest surface possible.
The varying amount of solder present on the component solder-pads may also make this finish less suitable for components with small and fine pitch pads.
Chemical Ni/Au (Electroless Nickle/Immersion Gold or ENIG)
Many customers choose this surface finish as it offers a flat surface, good solderability and acceptable shelf life.
It is a well-established surface finish for PCB’s having been introduced even before the requirement for Lead-free assembly. However, the ENIG process is complicated with a higher risk of defects (skip plating, black pad or interface embrittlement).
When using ENIG, the solder-joint is formed between the solder and the Ni layer of the NiAu surface, not with the underlying Cu. The Au is completely dissolved into the solder joint.
Also this interface seems to be considerably more brittle than with the HAL (SnCu) interface.
It is not recommended for applications which are subject to shocks, bending or strong vibrations.
One final point, it is the most expensive surface finish offered, however, for some applications, like key pads or wire bonding, it is the best solution.
Chemical Ni/Au Large Area (Electroless Nickle/Immersion Gold or ENIG)
Any copper area that is not covered by Soldermask and is ≥40% of the total copper area of an outer layer is classed as a larger area.
These will increase the cost of the PCB.
Chemical Ni/Au before Soldermask (Electroless Nickle/Immersion Gold or ENIG)
If this option is selected then the ENIG surface finish is applied before the Soldermask and all copper will be covered with ENIG.
Chemical Ag (Immersion Silver)
This surface finish is often a bone of contention, some love this finish, and some hate it.
It offers a flat surface with very good solderability. The solder-joint is created with the underlying Cu, since the Ag is dissolved during soldering process.
It all sounds good, but the ImAg is susceptible to sulphur dioxide (SO²) which tarnishes the surface and creates the AgS² layer. This layer affects adversely the solderability.
To avoid this tarnish, we pack the PCB’s in silver saving paper and hermetically sealed packing to avoid moisture and atmosphere SO² coming in.
In case of multi-step assembly, the partially assembled PCB’s should be stored in a sulphur-free atmosphere.
Depending upon the storage conditions the shelf life of this surface finish maybe an issue.
Other Available Surface Finishes
Hard Gold for Edge Connectors
Due to the need for edge connectors pads to be resistant to abrasion we recommend that these have an electroplated NiAu (Hard Gold) surface finish.
This finish is processed in a specialised bath that is only used for these type of connectors.
It is not possible to have this hard gold processed on other locations of the PCB.
Carbon
Carbon combines a high mechanical strength with a good electrical conductivity and can often be used as a substitute for gold on contacts.
It is printed directly on the Cu and used for switch contacts, foil keyboards and can enable the creation of cross-over conductors. It is resistant to HAL and soldering processes without showing practically any change in resistance.
Summary Table
Copper and the Board Edge
Copper and the Board Edge.
There are three options under our Advanced options heading which may be confusing:
- Copper up to board edge
- PTH on the board edge
- Round-edge plating.
Here’s how to sort them out.
Copper up to Board edge.
To avoid damage to the copper during the profiling operation we normally ask for a minimum distance between the copper features and the edge of the PCB. This distance is:
- 0.25 mm on outer layers with breakrouting
- 0.40 mm on inner layers with breakrouting
- 0.45 mm on all layers with V-cut scoring.
These figures are needed to accommodate industry-standard manufacturing and machining tolerances. For V-cut scoring it is also necessary to accommodate the V of the cutter.
Sometimes it is necessary to run a copper plane up to the board edge. In this case select “Copper to board edge”. There is no extra charge for this but it alerts our engineers to set up a different cutter speed.
“Copper to board edge” should normally only be used for planes and large copper areas where any slight damage to the copper will not impact on the performance of the PCB.
Tracks must not be placed within the minimum distance of the board edge where they could be damaged. Our engineers will raise an exception whenever they find tracks within the exclusion zone.
If we find pads within the minimum distance of the board edge, we will clip them back to restore the minimum copper-free space unless:
- the pads are part of an edge connector (usually with a bevelled edge)
- the pads are marked as “up to the board edge” in a separate mechanical layer
- the clipping is more than 25% of the pad surface in which case we will send an exception to the customer.
NOTE.
Copper to board edge cannot be combined with V-cut scoring.
PTH on board edge.
Also called “castellated holes”.
These are plated holes cut through on the board edge and used to join two PCBs either by direct soldering or via a connector. As the process requires extra steps, plated holes on the board edge are a cost-option.
Your data should clearly show the holes and the profile. Ideally include the information in a mechanical layer.
TIPS.
- There must be enough spare space on the edge of the PCB for us to hold the PCB in the production panel during manufacture. If you need castellated holes on all 4 sides, email us your design or proposed profile as early in the design process as possible. We can then confirm that it is manufacturable or suggest any necessary changes.
- You must have pads on top and bottom layer (and on inner layers where possible) to anchor the plating securely to the PCB.
- As a general rule the holes should be as large as possible to ensure good soldering to the mother PCB. We recommend 0.80 mm and above. Minimum end size possible is 0.5mm.
- All surface finishes are possible but our preference is for selective gold over nickel for the smaller sizes.
Round edge plating.
This means that most or part of the edge of a PCB or a cut-out is plated from the top side to the bottom side.
This may be to ensure a good ground to a metal casing or for shielding purposes.
To manufacture a board with round-edge plating we rout the board profile where the edge plating is required before the through-hole plating process. This involves extra process steps so round-edge plating is a cost option.
TIPS.
- Where the board edge is to be plated there must be copper on the top and bottom layer with a minimum width of 0.5mm from the edge of the board inwards.
- As we need to hold the circuit within the production panel during processing we cannot plate round 100% of the edge. There must be some gaps so that we can place rout tabs. If you need a very high percentage of edge-plating, email us your design or proposed profile as early in the design process as possible. We can then confirm that it is manufacturable or suggest any necessary changes.
- Indicate clearly in a mechanical layer where you need round-edge plating.
- Selective chemical nickel-gold is the only surface finish suitable for round-edge plating.
Tips & Tricks – Why Do Components Tombstone
|
during office hours. We will be more than happy to help.
Tips & Tricks: Avoiding Solder Escape/Wick during Reflow
|
Tips & Tricks – BOM & CPL Data
|
