As I touched on in our Class 2 v. Class 3 Part 1, there are many factors that determine whether a product falls under the IPC-610 Class 2 or 3. It is first necessary to determine the following:
- What is the end use?
- Is the reliability of the product vital to the maintenance of life?
- Was the PCB designed to adhere to these requirements?
One of the greatest factors driving increased costs when an assembly moves from Class 2 and Class 3 is inspection. As Circuitnet describes in their “Ask the Experts”, Class 2 vs. Class 3, “The major differences between Class 2 & Class 3 are found in component placement for surface-mount components, cleanliness requirements based on residual contaminants on the assemblies, and plating thicknesses as defined in plating through hole and on the surface of PCBs.” Greater attention to heel fillets, precise component placement, and the absence of contaminants, increases the thoroughness of inspection required, and in turn the amount of labor directed at the production and inspection of this product.
The IPC specifications pertaining to Electronic Assemblies are found within the 420 page manual that can be purchased through the IPC website.
Here are some common examples found in the assembly process that must be inspected to Class 2 or Class 3 based on the customer’s listed specifications.
Heel Fillets on Gull-Winged Leads:
The IPC guidelines state that in order for the solder of heel fillets to meet specs, there are minimum and maximum height requirements. For minimum Class 2 acceptance, “minimum heel fillet height is equal to solder thickness plus 50% thickness of lead at joint side.” For Class 3 acceptance, “minimum heel fillet height is equal to solder thickness plus thickness of lead at joint side.”
In order to achieve this, the pads must be appropriately sized (proper footprint) for the solder to have enough room to “wick up” the hole of the lead. Class 3 cannot be achieved if the air gap between pads is too wide. The solder cannot “wick up” the heel of the lead if the heel is not above the pad.
Ex: The back of the lead hangs off of the pad. The footprint has too large of an air gap, not allowing for an adequate heel fillet per IPC Class 3.
Barrel-fill for Through-hole Components:
Class 2 and Class 3 requirements differ on the amount of barrel fill required for through-hole leads. Class 2 requires a 50% barrel fill, while Class 3 requires a 75% barrel fill. Depending on the design of the boards, this may or may not be achievable. When designing the plated-through-hole (PTH), ACDi sets the goal of 15 mils over the diameter of the lead, as this leaves 7.5 mils on either side, allowing for paste to fill deeper into the barrel. The smaller the PTH, the more difficult it is to get the paste into the barrel. If the board is not designed to accommodate this, then Class 3 cannot be achieved during production. While there is no exact spec for PTH oversize to achieve Class 3, the smaller the gap surrounding the lead, the more difficult it will be to obtain the 75% fill.
Drill Breakout Requirement for Fabrication Class Specs:
Certain specs for Class 2 and Class3 are applicable at the design and fabrication levels of the PCB. The drill breakout requirement affects via size and the size of the annular ring. Class 2 allows for breakout from the annular ring, while Class 3 does not allow breakout. Because Class 3 allows not breakouts, annular rings must be oversized to accommodate tolerances with respect to drill sizes, drill registration, plating requirements, etc.
While the IPC-A-610 manual may not be the most thrilling read, familiarizing yourself with the specs laid out in it will help to ensure that you are communicating clearly what your product’s requirements are to your contract manufacturer, preventing any room for interpretation. It is crucial to consider during the design phase what implications the design and layout will have on the ability to produce the desired Class of product. THIS is why designing for manufacturability is an important goal; you will achieve the desired product and prevent delays during production from questions that could have been resolved on the front end.
Our design team will work with you to provide design-for-manufacturability assessments for your designs, as well as provide full design and layout services upon your request.