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Stencil Printing Small Apertures
Why size and shape matter
To understand the reason behind their extra cost — and to appreciate their importance in modern, complex PCB designs — we need a basic review. Below is a list of industry terms and their definitions, followed by an explanation of how blind, buried or filled vias are created in a fabrication shop..
INDUSTRY TERMS AND ACRONYMS
Printed Circuit board. This term is usually applied to the completed product consisting a fiberglass core with copper traces n’ spaces on the outer surfaces.
Trace n’ Space
The pattern applied to the layers of a PCB. Trace and space widths come from the design software and are contained in the “Gerber File” to the fabricator. They are usually designated by a thickness (in mils) of the trace followed by the thickness of the isolating space between the traces. For example, 5/6 refers to a 5-mil trace and a 6-mil space.
A thin piece of fiberglass laminate with copper cladding on both sides. The core has fully cured epoxy and is intended to be buried within a multilayer PCB structure. It is available in a wide range of thicknesses and of copper foil weights.
A photosensitive material applied to the copper foil of the core or the outer layers of a PCB to allow the transfer (photographically) of the computer-generated (CAD) image in the Gerber file to the copper surface of the core or outer layer of the PCB
A process of removing a base layer of copper leaving only the desired traces n’ spaces. The etching solution is masked from the traces n’ spaces by a photomask such that only the exposed base layer of copper is etched. This process continues to the surface of the laminate until all of the traces n’ spaces are un-shorted and there is no residual copper left between the traces.
The process in a multilayer PCB where the cores are sandwiched between layers of glass matt saturated with an uncured epoxy. The layers provide the insulating material between copper layers of traces n’ spaces and they also provide the “glue” that bonds the inner layers together.
Thin sheets of a woven glass matt that are saturated with a “B-stage” epoxy that is dry to the touch but uncured. When exposed to temperatures above the melting point of the epoxy, the B-stage flows around the copper circuitry (above and below) removing any air, filling all geometry, and bonding the surfaces together as it cures. Prepreg is available in a wide variety of thickness due to the size and type of glass used to weave the matt. The variety is further expanded by the types of epoxy resin saturating the matt. The resin varies depending upon the amount of flow that is desired when the epoxy is liquid.
The copper foil used by a fabricator is sold to them by weight. The weight is based on a uniform film of copper spread over 1 ft2 of surface. If 1 ounce of copper is uniformly spread over the 1 ft2, the thickness of the resulting copper is 1.4 mils (.0014”) thick.
NOTE: A customer request for “1 ounce copper” on the surface is due to a misunderstanding of how the term is actually applied. While the fabricator does purchase the copper foil by weight, (1 oz, 2 oz, etc.) once the fabrication process begins, this designation is no longer correct. The purchased foil is either eroded chemically and mechanically or increased by plating (electro or electroless). Fabricators ignore this customer misapplication of the terms and instead defer to the IPC–A-600 definition: “thickness after processing”, which re-defines ounces into a thickness of copper (For example, 2 Oz is defined after processing as being 2.2 mils instead of 2.8 mils.)
This is a fabricator term for the double-sided, copper-clad structure that comes out of a laminating press once the core, prepreg, and copper foil have all been pressed together and the epoxy has cured. To the untrained eye, the “brick” looks like a piece of double-sided, copper-clad laminate.
Sequential lamination refers to a repetition of the lamination process. If sequential lamination is employed, its purpose is to create an “intermediate brick” that is intended to be used for additional processes. In sequential lamination, multiple intermediate bricks or a combination of intermediate bricks and double-sided cores are bonded together. This process can be repeated multiple times depending upon the process capability of the fabrication shop. The typical number of laminations is 3, but a few very specialized shops can perform 5–6. These are rare.
The first lesson of stencil printing small apertures — don’t just accept the paste file from the designer.
It seems that each generation of assemblers forgets how stencil prints work. In my consulting, I am being told more and more often that the stencil file is sent to the stencil maker with no accompanying guidelines. Or, if there are guidelines, they are generic and steeped in tribal knowledge.
Let’s review: how a stencil print is made
To begin, squeegee action across the stencil surface fills the aperture with solder paste. Next, the paste contacts the underlying PCB pad, the flux in the paste adheres to the pad, and the stencil snaps away from the PCB pad. Finally, gravity and the flux adhesion to the PCB pad hold the solder paste slug as the stencil moves away. Modern printers are doing a wonderful job of filling the aperture with solder paste and cleaning the underside of excess paste. But beyond these machine-specific tasks, the stencil print is reduced to a battle of forces. The short list of these forces are:
The force of gravity acting on the mass of the solder “slug”
The adhesion forces holding the solder paste to the sidewall of the stencil
The adhesion forces attaching the solder paste “slug” to the underlying component padEach of these forces is further modified by the ball size, the flux-to-solder ratio, the viscosity of the flux, the speed of the “snap off”, and the method of preparation of the stencil aperture sidewalls. A basic review of the process shows why neglecting to modify the stencil files is a prescription for decreasing yields as the size of the aperture decreases.
Stencil printing large vs. small apertures
With large apertures, the adhesion to the pad (aided by gravity) swamps the other forces. But, as the area of the aperture is successively reduced, the adhesion forces holding the paste to the sidewall of the stencil begin to dominate. At some point, these sidewall forces inhibit the ability to print the solder paste through the aperture.
Process engineers of prior generations determined discovered the importance of maintaining a 1/3 to 2/3 minimum ratio between the surface area of the stencil sidewall (1/3) and the area of the opening (2/3). This is a rough rule of thumb, because it does not account for the effective adhesion of different surface finishes. When this ratio was generally accepted, the prevalent surface finish was HASL. Today, the really small apertures for 0201’s or 01005’s often have an ENIG finish, which is much “slicker”, and there are more options for the ball size in the paste.
While not always exact, the 1/3 ratio helps us to define a beginning rule for the smallest apertures that should be used with given stencil thickness. If we assume that the stencil is 4-mil thick and the aperture is square, then we can write a formula as follows:
- The area of the side wall is 4 times the area of rectangle of height 4 mils and length of “X”.
- Therefore, a single side of the aperture would be:
As= 4 * X, and the area of the four sidewalls would be 4 * 4 * X. Or, Ast = 16X.
- The area of the opening (being a square with equal sides) is X * X or X2.
Now, using the 1/3 to 2/3 ratio, we can create an inequality between the two areas and solve for “X”.
- X2 /16X >(2/3)
- X>2/3*(16) X>10.6 mils
Using the same formula, we can solve for the minimum dimension of a rectangular area of a 3-mil thick stencil:
- X>8 mil
It is necessary to convert these simple ratios back to an area, because not all apertures are square. The area tells us that:
- The minimum area for a 3-mil stencil is (8*8= 64 square mils ), and
- The minimum area for a 4-mil stencil is (10.6 * 10.6 =112.36 square mils)
In order to give these minimum areas a “reasonableness” test, we should look at the pad size of the smallest component currently in mass production — the 01005. Depending upon the data sheet that is referenced, the pad size for 01005 is approximately 11 mils on each side, or 121 square mils. Interestingly, the pad area for 01005 indicates that we cannot use a 4-mil stencil and expect to get reasonable results.
When printing small apertures, shape matters
If we carry this ratio of areas a little further, we must ask some questions about some of the common small pads for BGAs. Many of these are 9 mils in diameter.
The paste file from the designer will show a pattern of round 9-mil pads. We immediately have two questions:
- How should these be stencil printed?
- Is this the correct pattern for the stencil printer?
Round vs. square
The area of a 9-mil round aperture is [3.14 * (3.5) 2=38.465 square mils]. This is below the 64-square-mil minimum aperture opening for good deposition with a 3-mil stencil. However, if we were to change the round aperture to a square with 9 mils on each side, we would exceed the ratio needed for the 3-mil stencil, as this square pattern increases the area to 81 square mils.
We must further question why we are using a round aperture. Keep in mind that the solder paste is composed of round balls of solder. We may think of it as being more like toothpaste, but it is not. The deposition of the solder through the aperture must be in complete balls of solder. We cannot have a fraction of a ball.
The ball size of a given type of solder is actually a range, but here are some numbers for reference:
- The average diameter for a type-4 solder paste is 1.14 mils.
- The average diameter for a type-3 solder paste is 1.38 mils.
If we further consider that we cannot extrude a partial ball, our minimum area takes on further constraints. For maximum solder deposition, the aperture opening should have sides that are a multiple of the average diameter of the solder type to be extruded.
We have determined that for a 9-mil round pad, we should use a maximum of a 3-mil stencil with a type-4 paste. When we examine our chosen dimensions of 9 mil x 9 mil, (a rectangle instead of round), we find that our 9-mil side can only “line up” 7 type-4 solder balls with a diameter of 1.14 mils, which reduces the theoretical volume we are achieving by more than 10 percent. Therefore, our minimum aperture dimension for a 9-mil pad, with a 3-mil stencil and a type-4 paste should actually be 9.5 to 10 mils. This dimension allows us to have a solder matrix of 8 x 8 solder balls, but only two rows high (A ball diameter of 1.14 mils in an opening that is 3 mils high, cannot have more than two rows of solder balls.)
If we calculate the solder volume on this 9.5 x 9.5-mil aperture, we find that we have two rows of 8 x 8 or 128 total solder balls. This then yields a theoretical volume of 2.28 mil (height) X (8*1.14) 2 =189.6 cubic mils.
It is important to note the final volume of melted solder will be significantly less than the rows of solder balls stacked on top of each other. If the solder paste were indeed a “paste”, the volume in the 9.5 x 9.5 x 3 aperture would be 270.75 cubic mils.
Whew! So, with a type-4 paste, on a 9-mil round pad, using a 3-mil stencil and a square aperture of 9.5 x 9.5 mils, our max volume is only 70 percent of the theoretical (189 compared to 270 cubic mils).
Now, compare this volume to what would be achieved using a round aperture of 9 mils. We know from the volume analysis that we violate the ratio of aperture opening area to side wall area very significantly even with a 3-mil stencil. But it gets worse, since our solder paste is composed of round balls. If we draw a circle 9 mil in diameter and then fit inside it as many 1.14-mil solder balls as we can, we find we can only get 45 solder balls in a layer — and still only two layers deep (90 total solder balls).
The reason for this daisy chain of math is to show just how much the deposition of solder paste can vary from the expected deposition when we are pasting small parts.
Via-in-paid w/resin fill and copper cap
A lower cost alternative to the blind vias is a process that fills the thru-hole vias and vias in the BGA pads with a resin that replicates the coefficient of thermal expansion of the epoxy used in the prepreg. This process is especially viable when the trace n’ space density does not require blind vias, but high density BGAs are being employed.
In this process, the thru-hole vias and vias in the BGA pads are drilled and then thru-hole plated. This is followed by a panel electroplate to increase the copper thickness in the via holes and vias in the BGA pads. The thru-holes for components must be done in a second drilling and plating step. The plated via and BGA holes (no pattern has been applied) are then pressure-plugged with an epoxy resin that is Tg matched to the prepreg.
The resin is cured and then mechanically planarized to match the surface of the outer layer copper foil. The panel with resin-filled via holes is processed through the complete DSB process (drilling, plating, etching, etc.) for the component holes that must be left open for thru-hole pins. This process puts a copper cap over the resin dots in the via holes and the BGA pads. The end result is a BGA-pads-only outer layer that is easily stencil printed with solder paste.
Don’t use round apertures for small features, and be certain to fully examine the volume of paste that can be achieved with a given stencil design.
Stanley L Bentley, P.E. is the Senior Technical Advisor to RapidProto.com, a stand-alone supplier of Rapid Electronic Assemblies.