Part 9 - PCB Design Perfection Starts in the CAD Library

Part 9– SMD Bottom and Flat Lead Forms

Before we go deeper into the various component families, we need to clarify the component lead forms of today's component packaging technology and what is going to be eventually phased out and what is new and why. This is Part 1 of 2 for component lead forms. Part 2 will cover “Side & Bent” leads.

The pin (component lead) pitch and the overall body height are continually shrinking. This is why the SSOP and TSOP land pattern names have to be dropped from the standard. In this nomenclature, S = Shrink for Fine Pitch and T = Thin for low profile height.

If these two values are constantly changing then where is the line drawn? Whose part is Thin or Fine Pitch and by what measure? The Gull Wing lead has hit the limit at 0.40 mm pitch. Most assembly shops will try to convince you to swap that part out of your design for a larger pin pitch however, No-lead SON and QFN lead styles are being produced and manufactured at 0.40 mm pitch with no problems. The finer pitch parts have more I/O's and a smaller footprint with a much lower profile than J-Lead or Gull Wing packages, so it's obvious that the component industry is going to be no-lead or bottom only flat lead or side lead packages.

Let's review the bottom only and flat component lead forms. The BGA or Ball Grid Array has been around since the 1980s but the pin pitch started out with 1.50 mm and then quickly went to 1.27 mm (50 mils) for about 15 years. Then in the late 1990s, the 1.00 mm pitch BGA was introduced and every couple years a smaller pin pitch was introduced. Today 0.4 mm pitch BGA's are in every cell phone and 0.3 mm pitch BGA's are the next generation. See Figure 1.

Figure 1

For more information about BGA's, read my white paper "Metric Pitch BGA and Micro BGA Routing Solutions" located here - http://www.mentor.com/products/pcb-system-design/techpubs/

The next lead form in the "bottom only" category is the "Bump" lead. This is widely used in a package called "Land Grid Array" or LGA. The Land (pad) size can be the same as the maximum Bump Lead diameter. Via-in-pad technology can be much more forgiving with the Bump lead than BGA voids due to a dimple in the Land after the plug and plate process. This lead form is also highly compatible with lead-free solder. See Figure 2.

Figure 2

The next Grid Array lead form is the "Bottom Flat" and is also used in Land Grid Array (LGA) component packages. Linear Technologies is the leading provider of Bottom Flat Lead LGA packages. This lead form is also highly compatible with lead-free solder alloys as there is no requirement for wetting (flow) properties in the solder.

The other Flat No-lead is referred to as a "Pull-back" or "Bottom Only" lead. We can also categorize the Pull-back Lead Small Outline No-lead (SON) and Quad Flat No-lead (QFN) component packages with this solder joint goal as a slight periphery land is required to allow the solder to move from under the lead to the periphery to surround the protruding lead for a solid solder joint.

The solder joint goal is a periphery land around the terminal. Pull-back or Bottom Only leads come in three lead shapes -

D-Shape (or Bullet in some CAD tools)
Square
Rectangle

This lead style has the same solder joint goals as the Bottom Only “Bump” LGA lead. See Figure 3.

Figure 3

The next Grid Array component lead is the "Column". Actel and Xilinx are the leading manufacturers for this lead style. You will not find any pin pitches smaller than 1.00 mm for the Column Lead. The Land must be slightly larger than the column to form a good solder joint. See Figure 4.

Figure 4

The last SMT Grid Array is the newest lead form in the industry is the "Pillar Column". Recently introduced by Actel, this component lead has much promise for an improved solder joint. But time will tell how long this one will last. See Figure 5.

Figure 5

"Flat Lead" components are coming on strong. These are the SODFL (Small Outline Diode Flat Lead) 2 leaded components and the SOTFL (Small Outline Transistor Flat Lead) packages that come in 3, 5, 6 and 8 lead components. Both of these component families are the direct replacement for the Gull Wing Lead SOD and SOT-23 packages. See Figure 6

Figure 6

"Flat No-lead" is used in the SON (Small Outline No-lead) with terminals on 2 sides and QFN (Quad Flat No-lead) with leads on 4 sides. The most common SON & QFN today is the "Edge" lead, where the component lead starts under the component and goes out to the component body edge. This solder joint goal requires a Toe, Heel and Side solder fillet where the toe joint is visible for inspection. See Figure 7.

Figure 7

The "Flat Thermal" lead comes in a DPAK where the signal pins and Gull Wing and the thermal lead is flat. The "Flat Thermal" lead is also used as the heat sink for SON, QFN, SOP and QFP packages. It is usually embedded in the plastic component body and therefore the solder joint goals are usually 1:1 scale for the maximum component lead size and Land size. See Figure 8

Figure 8

The last component lead form in the list is the "Under Body Outward L". This lead form is used for Aluminum Electrolytic Capacitors and 2-pin SMD Crystals. This lead form has 2 different solder joint goals that are based on the component height. Once the component height exceeds 10 mm, the solder joint goals have to be more robust. See Figure 9.

Figure 9

Coming Up

Additional brief topical articles will appear in future newsletters. You can also read more detail in my blog, which can be found at:http://blogs.mentor.com/tom-hausherr/

Written by Tom Hausherr CID+

EDA Library Product Manager

Mentor Graphics Corporation

Reprinted by permission from iConnect007