The Chip Package is the most widely used in the electronics industry. Chip components have an average of 80% of the total parts on a typical PCB assembly. Here is an image of the average chip terminal lead form.

Let’s compare the differences between IPC-J-STD-001 and IPC-7351B standards.

IPC-J-STD-001 uses 3 classes to define solder joint goal acceptability for various electronic products.

CLASS 1 – General Electronic Products

Includes products suitable for applications where the major requirement is function of the completed assembly. Toys

CLASS 2 – Dedicated Service Electronic Products

Includes products where continued performance and extended life is required, and for which uninterrupted service is desired but not critical. Typically, the end-use environment would not cause failures. Computers & Phones

CLASS 3 – High Performance Electronic Products

Includes products where continued high performance or performance-on-demand is critical, equipment downtime cannot be tolerated, end-use environment may be uncommonly harsh, and the equipment must function when required, such as life support or other critical systems. Military & Medical

IPC-J-STD-001 defines the requirements for electronic assemblies. Land pattern footprint pad sizes and locations play a significant role in meeting these requirements. The IPC-7351 guideline falls under J-STD-001 to define the solder joint goals for Toe, Heel and Side values and the land pattern and pad stack naming conventions.

The footprint pad size is determined by the terminal lead size, the terminal lead tolerances, the manufacturing tolerances for fabrication and assembly and the Toe, Heel and Side solder joint goals as defined in the IPC-7351 guideline.

The Toe solder joint goal is the most important for the Chip Terminal Lead and the J-STD-001 Minimum Fillet Height for a Chip Component for the Toe Goal is:

The F dimension for Toe Fillet for Class 1 & Class 2 Fabrication is:

• Wetting is evident on the vertical surface of the component terminal

The F dimension for Toe Fillet for Class 3 Fabrication is:

• (G) + 25% (H) or (G) + 0.5 mm, whichever is less

Example: For Class 3, let’s say that the 0805 Chip Capacitor is 4.00 mm height. 25% of 4 = 1.00 mm. But the IPC-J-STD-001 standard says the 0.50 mm is an acceptable solder fillet height because it’s less than 1.00 mm. But what about an 01005 chip resistor with a 0.13 mm height? Using a 0.075 stencil thickness, J-STD-001 recommends a Toe fillet of (G) 0.07 + 25% (H) 0.03 mm = 0.10 mm. But (G) is actually less than 0.05 mm after reflow.

If the Toe pad length is too long, there will be excessive solder. When going into a reflow oven, if there is excessive solder and pin 1 hits the heat first, the excessive solder will melt and pull the chip up into a tombstone position. To eliminate tombstoning, the pad Toe must not exceed the J-STD-001 requirements.

Here are the IPC-7351B tables for all chip packages including Resistors, Capacitors, Diodes, Inductors, Antennas, Ferrite Beads, Filters, Fuses, LED’s, Thermistors and Varistors for 3 density levels, Most, Nominal and Least pad size.

Note that for all chips longer than a 0603, the Nominal Toe value is 0.35 mm for 0603, 0805, 1206, 1210, 1812, 2010 and 2512, regardless of their height. And for every chip package less than 0603 the Nominal Toe value is 0.20 for 03015, 01005, 0201 and 0402. Also note that the difference between density levels for chips larger than 0603 is +/- 0.20 mm for Most and Least density levels and chips smaller than 0603 have a +/- 0.10 mm difference between Most and Least density levels.

PCB Libraries, Inc. has a philosophy that every chip size should have a unique Toe value that is aligned with J-STD-001 mathematical model for the acceptable solder joint.

Here is a table for the default values in Library Expert, but all of these settings are user definable to best meet your assembly attachment requirements. Also note that there is a +/- 0.10 mm between Most and Least density levels for all chips.

PCB Libraries, Inc. refers to this concept as “Incremental Pad Stacks” for the sole purpose of achieving the best solder joint goals in the assembly process. You can download the entire Library Expert Solder Joint Goals Excel spreadsheet here – http://www.pcblibraries.com/downloads" rel="nofollow - www.pcblibraries.com/downloads or if you’re using Library Expert, the Excel spreadsheet is on your computer in this folder - C:\Program Files (x86)\PCB Libraries\Library Expert 2019\Documents\Library Expert Solder Joint Goal Tables.xlsx

Note that there are negative values for some Heel and Side solder joints. This is to compensate for the Fabrication and Assembly Tolerances (approximately 0.04 to 0.05 mm) in the IPC-7351 mathematical model. The Fabrication and Assembly Tolerances have been part of the IPC mathematical model since 1987. In the past 33 years, the fabrication and assembly processes have been improved so much that these tolerances are no longer applicable to todays advanced machines. Also, most fabrication shops swell the outer layers to compensate for their etching process. Adding an additional fabrication tolerance to the pad size calculation is a “Double (duplicate) Tolerance” and is unnecessary.

We recommended that the Fabrication and Assembly Tolerances be turned off to 0.00 and all negative solder joint goals also be turned to 0.00. This would give the user much better control of the pad stack calculation. But this is an individual choice that every company must make on their own. We hope that the unreleased IPC-7351C will make this adjustment to the mathematical model.

It’s also important to note that Lead Solder flows much better than Lead-Free Solder. Here is an image of a chip with lead solder. You can visually see that the solder flows from the pad Toe up the side of the chip terminal. Also note that the pad Heel is 0.00 and flush with the chip terminal.

Here is an image of chips using Lead-Free solder. Most of the solder just stays on the pad and does not flow up the side of the chip terminal lead like Lead Solder. Also, this example illustrates excessive pad Toe. This consumes important PCB real estate, wastes solder and does nothing to add to the solder joint quality.