What is Stencil in SMT Design ? A Simple Guide To SMT in Embedded Hardware Design
by cawan (cawan[at]ieee.org, chuiyewleong[at]hotmail.com)
Embedded hardware design in most of the times applying Surface Mount Technology
(SMT). SMT is about to "stick" all the Surface Mount Devices (SMD) such as resister,
capacitor, inductor, diod, integrated circuit (IC), and etc. Each SMD has respective
packaging in order to fit itself onto the solder pad of printed circuit board (PCB).
For resister or capacitor, its packaging is normally expressed in the form of length
and width, such as 0805 or 0603. For the case of 0805, it means the SMD's length and
width is 0.08 inch (2.0mm) and 0.05 inch (1.2mm), respectively. On the other hand,
the dimension on PCB to fit the 0805 SMD should be 0.12 inch (3.05mm) length and
0.05 inch (1.2mm) width. Of course the dimension of solder pads are clearly defined
by the standard accordingly. For the case of IC packaging, it is normally about QFP
or BGA. QFP has all the pins aligned properly at 4 edges of the rectangular IC.
However, BGA deploys all the pins as ball grid underneath or at the bottom side of
the rectangular IC. Both of them (QFP and BGA) have their own manufacturer dependent
definition about the dimension of pin length, width, and pitch. Besides, it is
important to note that BGA can support higher clock speed than QFP due to the reason
of shorter route from core to pin (better known as ball) interconnect. So, BGA is
mostly applied in DDR-RAM or SoC which is operating in high-speed or high clock speed.
Well, with all the solder pads exposed for soldering process later, the rest are
covered or protected by solder mask. So, it is impossible to solder at the area of
solder mask. In fact, for SMT, it is unusual to perform soldering process manually.
Instead of it, automated PCB assembly is a better or more appropriate approach. This
is because it is rather hard to minimize the mistakes such as component misplace,
not proper solder, over solder, or lead overflow and cause short circuit on board.
Try to imagine, is it logical to expect a human being to solder 20 types of 0603 and
0805 mixture of SMDs where the total quantity is about 300 on a 9cm x 6cm PCB without
any mistake ? Yes, it is almost impossible. As a better approach, the assembly process
should be automated by machine, and the process is normally known as PCBA. The PCBA is
about to automate the process of placing and aligning all SMT components onto the PCB
and solder all of them together. Well, it involves 2 steps, first, placing and
aligning the SMT components; and second, to solder all of them together. Let's start
from the first step. In order to put all the SMT components properly onto the PCB, we
need a very precise pick-and-place process, but this shouldn't a problem. The real
concern is about how to make sure all the SMT components do not misalign on the PCBA
process. Yes, we need some kind of "glue" to "stick" them onto the PCB. Of course, in
PCBA process, we can't expect UHU or any superglue can work well. Instead, we need
flux to do the job. Just assume flux is some kind of chemical to improve the soldering
process by making the solder lead has good join to the solder pad. So, we need some
mechanism to put flux onto all the solder pad of the PCB now. In this case, we need
stencil. Stencil is a piece of stainless steel which is almost the size of the PCB.
On stencil, at the position of each solder pad, it has a hole with respective
dimension of the solder pad, and it is made by laser. Hence, the appearance of a
stencil is about a piece of stainless steel board which is full with holes. So, by
putting the stencil onto the PCB, it is simple to put flux onto all the solder pads
of the PCB in one-shot. Well, while all the solder pads with flux, it is also known
as solder pastes. As what the meaning of "paste" is indicated, it is simple to perform
pick-and-place of SMT components onto the PCB and also to ensure the components are
properly aligned on the process of PCBA. Lastly, the second step or the last step of
PCBA is about to put the PCB with all the SMT components "stick" onto the respective
solder pads into oven. So, once the solder lead begins to melt, the mixture with flux
will join all the SMT components onto the solder pads accordingly.