SMT Magazine Column:
The Short Scoop Series
The Short Scoop: Stencil Printing in PCB Cavities
This month's SMT Magazine column discusses whether a single-thickness electroform stencil can print flux or solder paste into a recessed area ona PCB for an embedded flip-chip with a cavity depth of 14 mils (350 μm), which is becoming a requirement for many components.
Normally, to print on both levels requires special stencil and squeegee blade designs. A two-step stencil process is often used. We wanted to see if one stencil could be used instead. Using just one stencil to print both levels at the same time saves time, money, and resources.
Find out the results of our stencil printing in PCB cavities here...
The Short Scoop: Printing Two-Level PCBs in One Step
with a 3D Electroform Stencil
The requirements for two-level PCBs with components on both levels have seen a recent increase. Stencil printing on both levels requires special stencil and squeegee blade designs. Recently, we participated in two experiments. The purpose was to determine if a 3D electroform stencil, in just one printing step, could be used to print a two-level board with cavities. The tests consisted of:
- Printing solder paste for .3μm μBGAs with pads on two levels (steps) of a flexible PCB separated by 7 mils (175μm).
- Printing flux and solder paste into a recessed area on a PCB for an embedded flip-chip with a cavity depth of 14 mils (350μm).
The Short Scoop: Improving Stencil Printing Results
Discusses some possible answers
to the frequently asked question, “Why am I getting poor printing results?” There are a myriad of causes of poor print
performance. The problem may stem simply
from an inferior or worn-out stencil, which
is typically the first place people focus when
troubleshooting. However, the issue may also
be caused by an improper aperture design or
stencil thickness. Additionally, poor print performance
might not be caused by the stencil
itself, but rather an improper printer set-up, a
non-optimal squeegee blade, or the rheology of
the solder paste being used during processing. Read the article to see a list of some of the problems users
encounter and possible solutions.
Learn how to improve stencil printing results ...
The Short Scoop: More Stencil Questions (and the Answers!)
What are my fiducial choices and how
do they differ?
Yes, stencils require fiducial markings to
make sure the stencils are aligned correctly to
properly print the pattern, as most people know.
What they might not know is that there are actually
several types of fiducials, and when you
select a stencil, you must choose one of them.
Find out the rest of the answer to this and other questions ...
The Short Scoop: Selecting a Stencil Frame
For a stencil, the frame primarily provides a rigid support structure that allows the stencil to be used with your printing equipment. Without the frame, the stencil is no more than a thin foil with limited use due to the flexture of the stencil when left unsupported. The frame enables the foil to be stretched taut so that the stencil can gasket to the board, and thus solder paste will release properly from the stencil onto the circuit board.
Learn more about selecting a frame ...
The Short Scoop: Screen Printing Solutions for Small Die and Precision
Find out the challenges and opportunities for improving precision results that lead to a unique adaptation of an AccuScreen stencil. The challenge was printing solder paste for QFN devices, which present several assembly problems. QFNs have a metal pad on the underside of the part for grounding and heat conduction. The packages are very small and light. The QFN leads and ground plain conductor are flat and in the same plane on the bottom of the package. Read this case history article ...
The Short Scoop: 10 Common Stencil Questions
What type of stencil do I need? How much will it cost? These are some of the questions we answer in this month's column from Rachel Miller-Short. The need to satisfy more rigorous printing requirements due to finer pitch components leads to a natural hierarchy of stencil technologies based on the application. A few general stencil technologies are available on the market today. They include stencils that are laser-cut on some form of stainless steel material, laser cut stencils on 100% nickel foils, and electroformed stencils for the most challenging applications. Check out more stencil questions and answers ...
The Short Scoop: Electroformed Stencils
Electroformed stencils have become much more of a necessity as pitches and components become smaller. They are typically used for active component pitches below 20 mils, for 0201 and 01005 chip component applications such as SMT, μBGAs, flip chip, and also for wafer bumping (12 mil to 6 mil pitch). Read more about Electroform Stencils ...
The Short Scoop: Why the Customer-Stencil Manufacturer Relationship Matters in 2014
When SMT Magazine editors asked their columnists to think ahead and discuss the technologies and changes that will lead the way in our field in 2014, my thoughts and predictions naturally turn to stencils. The big change is in the customer-stencil manufacturer relationship, requirements, and expectations. Read full article here ...
The Short Scoop: Stencils for QFNs
Quad flatpacks, no leads (QFNs) and dual flatpacks, no leads (DFNs) are becoming more popular component packages, in part because they are ideal for small, hand-held devices. As stencil manufacturers, our thoughts immediately turn to the question, how do we design and manufacture a stencil for these devices? Read about stencils for QFNs here ...
The Short Scoop: Step Stencils - Applications and Solutions
In my last Short Scoop, (The Ins and Outs of Step Stencils) you heard a little about step stencils--what they are, why demand is increasing, different types, and what to look for. In this column, we explore step stencil applications and solutions. Read more about step stencil applications ...
The Short Scoop: The Ins and Outs of Step Stencils
The demand for step stencils is increasing, but why? The answer: shrinking components. In the early days of SMT assembly, step stencils were used to reduce the stencil thickness for 25 mil pitch leaded device apertures. However, as SMT requirements became more complex and, consequently, more demanding, so did the requirements for complex step stencils. Read about step stencils ...
The Short Scoop: Squeegee Blades
One often overlooked component of the soldering process is the squeegee blade, which is critical in solder paste application. The technology of the seemingly simple squeegee blade is as important as the stencil when it comes to the printing process. Learn about squeegee blades here ...
The Short Scoop: Introduction to Stencil Printing
Approximately 50% of defects that occur on an SMT assembly line can be attributed to the printing process. Doesn’t it seem odd, then, that instead of focusing on quality and experienced technical support, stencils have become a commodity and are being sold by a broad range of stencil suppliers, sometimes solely on the basis of price? Read more about stencil printing ...