Typically as the stencil aperture / pad dimension reduces, a smaller solder particle size may be needed.

Rules of thumb

• A minimum of 4-5 solder particles should span the width of the stencil aperture (for Type 3 powder, solder particle dia is 25-45 microns; for Type4 powder, solder particle dia is 20-38 microns)
• Type 3 powder is used for (a) 20 mil pitch CSPs & above 16 mil pitch QFPs & above; Type 4 is typically used for anything smaller

Print objectives for smaller stencil apertures:

1. maximize print transfer efficiency (volume of paste transferred from the stencil to the PCB)
2. achieve consistent print volumes with minimum deviation both at time zero as well when the paste sits on the stencil (response-to-pause)

1. 80% of SMT defects aredue to the print process
2. inconsistent print deposits for smaller apertures have a direct bearing on defects such as head-in-pillow & improper solder coalescence

Apart from solder particle size, the flux in the solder paste is crucial for achieving good consistent prints- the flux rheology is key in preventing paste getting trapped in the stencil aperture and instead get transferred onto the PCB

A good flux with Type 3 powder can be better at paste transfer than a bad flux with Type 4 powder

Karthik Vijay Technical Manager, European Operations, Indium Corp. Karthik Vijay is the Technical Manager for Indium Corporation's European Operations. He is based in the UK and is responsible for technology programs and the technical support function for Indium Corporation's customers in Europe. His expertise is focused on solder paste, engineered solders, thermal interface materials, and semiconductor-grade electronics materials.

Type 3 solder paste is generally considered to be the industry standard that will work for most printing applications. The use of Type 4 is really only needed for particularly fine-feature printing, where the Type 4 will produce better release and a more consistent volume deposit.

The breaking point for Type 3 solder paste in printing applications is generally considered to be in the range of a 9-mil stencil aperture width for most pastes, although other variables such as paste chemistry, stencil thickness and other printer parameters can impact this somewhat. This means that Type 3 paste will generally work well down to an aperture with an opening of 9 mils or greater. For apertures that are narrower than 9 mils in width, the user is best off using Type 4 paste to assure consistent release from the stencil.

The rule of thumb that is appropriate in this case is called the "Five-Ball Rule." This rule says that you need to be able to fit five of the largest powder particles across the narrowest aperture in order to achieve good stencil release.

Since Type 3 paste has a specification of 25 - 45 microns (roughly 1.0 - 1.8 mils), you can fit 5 of the largest powder particles across a 9-mil stencil aperture (5 particles x 1.8 mils per particle = 9 mils). If the aperture were any smaller than 9 mils, the Type 3 paste would fail the Five-Ball Rule and the user should consider Type 4 powder instead.

Brian Smith General Manager - Electronic Assembly Americas, DEK International Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.

The major reason to choose a Type-4 powder over Type-3 is to improve printing in small apertures. As a general rule of thumb, we recommend using 16mil pitch, and micro BGAs from 10mil and below and as a cut-off point for type-3 powder. In typical design rules, pad width is roughly half its pitch, so a 16mil pitch component will have a pad width of approximately 8 mil.

We have found that the stencil can have a greater impact on improved printing than powder. Some of the new Fine Grain foils allow for paste release down to a .45 surface area ratio. Work closely with your stencil provider to ensure they are able to provide you with the latest technology and best solution to meet your needs.

Mike Scimeca President, FCT Assembly Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.

The normal rule of thumb is the stencil aperture should be at least 5 times the average solder sphere in the solder paste. Type 3 solder paste has an average solder sphere of 36 microns. So any aperture smaller than 180 microns (7 mils) should use type 4 solder paste. The average solder sphere for type 4 is 30 microns. So any aperture smaller than 150 microns (6mils) should use type 5 paste.

Bill Coleman Vice President Technology, Photo Stencil For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.