Hot Air Solder Level (HASL)

The HASL process consists of a pre-clean, fluxing, hot air leveling, and a post-clean. Pre-cleaning is usually done with a micro-etch. However, the usual persulfate or peroxide micro-etch is not common in the process. Dilute ferric chloride or a hydrochloric-based chemistry is favored for compatibility with the fluxes that are applied in the next step. Fluxes perform the following functions:

• Provide oxidation protection to the precleaned surface.
• Affect heat transfer during solder immersion.
• Provide oxidation protection during HASL.

Higher viscosity fluxes provide better oxidation protection and more uniform solder leveling, but reduce overall heat transfer and require a longer dwell time or higher temperature. A balance in flux use must be struck between better protection with high viscosity fluxes and superior heat transfer with lower viscosity fluxes (ref. 38).

Hot air level machines consist of a transport mechanism that carries the panel into a reservoir of molten solder (460°F, 237°C), then rapidly past jets of hot air. All areas of exposed copper are coated with solder and masked areas remain solder-free. Boards are then cleaned in hot water, the only step in the SMOBC process where lead may enter the wastewater stream, albeit in very small quantities. Once cleaned, the panels may again enter the screening area for optional nomenclature screening, or proceed directly to the routing process.

Copper, flux and other impurities build in concentration in the solder pot as panels are processed through the hot air leveler. These impurities can be removed to some degree by performing a procedure known as drossing. From the hot operating temperature, the temperature is reduced to 385°F (196°C) and the machine sits idle for 8 to 12 hours. The impurities will float to the surface of the solder where they are scooped out and placed in a dross bucket. This material can be returned to the vendor for reclamation of the metals. Some manufacturers go for years without changing the solder, they dross and make additions. When the time comes to change over the solder, vendors will issue credit on the purchase of new solder as long as the old solder is returned to them for processing.

The acid pre-clean will have some copper in solution and can be treated conventionally. The waste flux is collected and is sent off-site for treatment.

Printed Circuit Board Material Selection

Material selection for the construction of a printed circuit board is a critical step towards the final product.

Product Application Recommendations

Application recommendations for selection of a specific type of material is based upon consideration of the following:

• Material Suitability
• Best in Class Performance
• Cost / Performance Ratio

Key Questions:

• Will the (PCB / PWB) printed circuit board successfully survive the reflow process?
• Will the (PCB / PWB) be fit for use and have maximum reliability in use after the reflow process?

Specific Material Selection Suitability

The application suitability of a specific type of material is based on consideration of the following parameters:

• Tg - Glass transition temperature
• Td - Decomposition temperature
• Z axis expansion
• PWB Processability
• I.S/T/ performance
• T260 and T288 performance
• Solder float survivability
• Thermal cycling performance
• And additional data

Printed Circuit Board Substrates

The majority of today’s printed circuit boards are made up of layers, also referred to as multi-layer printed circuit boards. Most circuit boards have between 1-16 layers and are separated by a substrate which is a laminated insulator that separates the circuitry design. Each layer of the circuitry is connected to each other through a series of drill holes.

Theses holes know as VIAS are plated holes that connect copper tracks from one layer of the printed circuit board to other layers they can be plated or have a rivet . There 2 types of vias used in multi-layer printed circuit boards:

• Blind Vias: This is a vias that is visible on one side only.
• Buried Vias: This is a vias that is not visible on either side

Printed Circuit Board Substrates

Substrates can be classified into five types, each having a unique set of characteristics for specific applications.

FR-2 Paper Printed Circuit Board Substrate

• This lower grade of substrate is a made of impregnated paper, also called Phenolic, that is easy to machine over a fiberglass material substrate. The “FR” refers to the term Flame Resistant. This type of substrate is usually found on more inexpensive consumer electronics.

FR-4 Fiberglass Printed Circuit Board Substrates

• Fiberglass substrates are made up of a woven fiberglass material and impregnated with a flame resistant material. The material is rigid and can also be drilled cut or machined but due to the abrasive nature of the fiberglass, tungsten carbide tools are needed. An FR-4 is a stronger substrate compared to an FR-2 and is more resistant to cracking or breaking and are usually found in higher end electronics.

RF Radio Frequency Printed Circuit Board Substrates

• RF substrates are comprised of low dielectric plastics and used in printed circuit boards for applications in high power radio frequencies. Although the substrate has poor mechanical properties it has exceptional electrical performance properties.

Flex – Flexible Printed Circuit Board Substrates

• Not all circuit boards use rigid core materials. Some are designed to be very flexible or slightly flexible called flex circuits. Thin and flexible plastics and or films are employed as substrates. The manufacturing process is more difficult than using rigid substrates, but offers benefits that cannot be achieved with rigid substrates such as saving space by bending the circuit board to fit a particular space or where repetitive movement requires a flexible layer.

Ceramic/Metal Core Substrates

• Power electronics demand a low-thermal resistivity substrate. A ceramic core or metal core substrate provides the necessary characteristics to handle larger copper tracks and the high electrical currents used with these type of circuit boards.

Multilayer PCB Prototyping

Overview

The primary intent of fabricating a multilayer printed circuit board is to provide one more degree of freedom in the selection of suitable routing paths for complex and./or noise sensitive electronic circuits. Such considerations as layer to layer impedance aside, what we are really trying to do is stack two or more circuits on top of one another and establish a reliable set of predetermined interconnections between them. There are a number of well established techniques that are routinely used by the printed circuit industry to accomplish this. However, they all suffer from their reliance on a large number of chemical processes (as many as 15) to condition the substrate, activate the through-holes and electrolyticly plate copper between the adjacent layers. The following method reduces this dependence on "wet chemistry" to a single acid copper electroplating bath and relies on a small set of easily mastered processes to eliminate the need for much of the complexity normally encountered in multilayer board prototyping.

The technique begins with a departure from conventional processing in that all of the layers are drilled and plated before they are laminated together. The two innermost layers will comprise a conventional (if somewhat thin) two sided PCB and the various outer layers will be fabricated as separate single sided PCBs. Prior to lamination, the inner layer board will be drilled, plated through, imaged, developed, and etched. The drilled outer-layers (signal layers) will be plated through in such a way that uniform donuts of ductile copper are formed on the underside rims of the through-holes. (The underside is the side without copper foil) During lamination of the various layers into a composite multilayer, these donuts bite into pads on the surface of the opposing layer to form gas-tight, wave-solderable interconnections. Sounds pretty easy so far, eh?

Getting ready

Equipment needed

In addition to a conveniently laid out work area with good ventilation, a source of hot and cold water, and adequate UV proofed lighting, you will need:

• high-speed drill (or a good drill press with min. 3500 RPM capability)
• Some board shops will drill boards for much less than they charge to fabricate a finished PCB. Call around, it might be worth your while. Be sure, however, that you clearly understand all of the costs involved. Ask if there are things you can do to the design and/or the CAM file that will lower the cost to you.
• If you will be hand drilling your substrates, press the button {hyperlink to hand drilling/formatting, on-line access only}
• electrolytic copper plating cell
• dry-film laminator or photopolymer compatible silkscreen
• ultraviolet exposure unit
• photopolymer developer
• copper etcher
• laminating press

Before you start

Although the techniques presented below will work equally well on most of the common materials used in PCB substrates (e.g. FR4, FR2, polyimide), the description will specifically deal with various forms of FR4, the most commonly used PCB material. Key considerations include:

1. If possible, panelize your design so that you get more than one board out of your panel. If this is your first multilayer board, choose a small, simple design that will allow you to make at least two individual boards. That way, if one is defective, you might still end up with one good one. As you gain experience, this is less of a consideration but, making more than one copy of your design is always a wise practice.
2. Select the substrates for the various layers based on the impedance and signal isolation requirements of your design and the physical limits placed on the total thickness of the laminated panel.
3. For a four layer design with a finished thickness of 0.063" (1.6mm), copperclad with 0.0007" (17) copper foil on both sides (so called half ounce/half ounce or "half over half" copperclad) and a substrate thickness of 0.025" (.64mm) is a good selection for the innermost PCB (designated .5/.5 0.025" FR4).
4. The outer layers will be constructed from copperclad with half ounce foil on one side and a substrate thickness of 0.017" (0.43mm). The substrates used in these outer layers are generally comprised of laminae of glass fabric held together with a partially cured resin that will remelt during multilayer lamination. Remelting causes the resin to become VERY tacky and adhere to the inner layers. This is the bonding mechanism that lies at the heart of multilayer fabrication. Copperclad based on these partially cured resins is referred to as "B stage" laminate. B stage is also now available with a polyester release liner lightly adhered to the resin side to protect the bonding surface during all pre-lamination processing.
5. The thickness of substrates used in PCBs with a higher layer count should be scaled accordingly (if the finished thickness is constrained to some preset value). One of the real advantages of the following technique is that, assuming that you can drill all of the layers simultaneously, in a single stack, you can pretty much stack as many layers as you want.
6. Set aside all of the drill bits that you will need, with spares if possible. This is especially helpful if you will be manually drilling the board. Hand drilling is hard enough without having to interrupt the work flow to go look for a drill bit. There is a fairly substantial body of scientific evidence that states that many of the most notorious mass murderers in history started out as mild mannered PCB drillers.
7. You will need at least 2 ea. 0.125"(or 3mm) dowell pins. If these are unavailable, broken 0.125" (3mm) shank carbide PCB drills will serve admirably. Make sure that you grind down any sharp edges to avoid cutting yourself and your counter tops during drilling.