The S-100 hobby is experiencing a surge in popularity in the first quarter of the 21st century. Although participation is a tiny fraction of what it was in the 1970s and 1980s, reproductions of legacy boards as well as brand new designs are now available. Production of PCBs is less centralized and more ad hoc than in the 20th century, which is a good thing for this small community. While there are a handful of individuals providing PCB fabrication services to the community, I often see individuals struggling to figure out how to place an order for a PCB that no one seems to stock. Granted the design files are open-source and readily available, they most often do not provide fabrication details for the novice. This article attempts to fill in these gaps.

We will begin the discussion with the basics and topics that apply to reproducing both legacy 1970s PCBs and modern layouts. They are different. We will then focus on the particulars regarding reproduction legacy PCBS. We end the discussion with suggestions for new designs.

In the meantime, here are some common terms we will use:

Hard Gold: Gold over Nickel, as it was commonly referred to. Typically 120µ" Nickel is electro-plated onto the copper, followed by 10µ" Gold. This is the “reference standard” for card edge connector “fingers.” It provides the most durable conductor surface for card edge connectors.
Legacy PCB: Designs that were manually taped up without significant copper pours (ground and/or power planes). They are usually identified with “heavy” traces serving power to rows or columns of ICs. Power distribution is generally the issue with these PCBs. It is important to remember that 2 ounce copper was par for the course in those days, as opposed to 1 ounce copper nowadays.
Modern PCB: Designs with ground and power planes that were laid out on a CAD system. Nearly all of the boards on http://s100computers.com fall into this category. You may occasionally find a PCB from the 1980s laid out this way. Multilayer PCBs also fall into this category.

Here are some links to Wikipedia articles for standard terms: ENIG, HASL

Conclusions

To make life easier for readers in a hurry, we begin with the end. What follows the conclusions are the supporting details and thought processes. The parameters that are common for all S-100 PCBs are:

Board type: Single piece. It doesn’t make sense to panelize large through-hole assembled boards.
Size: 10 × 5 inch or 254 × 127 mm (approximately). There is a provision for 10 × 10 inch boards in the IEE 696 specification, but is rarely used.
Material: FR-4. Choose the highest temperature grade (FR4-TG) offered without increasing the price.
Thickness: ¹/₁₆ inch, 0.625 inch, or 1.6 mm.
Edge connector: Yes, with 45° beveling and hard gold. See Edge Connectors for more information, as this parameter affects the cost of the PCB more than anything else.
Surface finish: HASL with lead. This is almost always the cheapest and best option. If they don’t offer leaded, get HASL lead free.
Finished copper: Legacy, 2 ounce. All others 1 ounce.

Edge Connectors

Perhaps the biggest and most important decision when fabricating S-100 PCBs is the edge connector. The earliest Altair PCBs used tin/lead plated fingers. After lots of reliability problems, MITS and everyone else agreed on what’s called Hard Gold for the connector fingers. Because card edge connectors are a pretty rare thing these days, and other factors, putting hard gold fingers on a PCB is expensive.

If you are having a PCB fabricated that will go in and out of the backplane more than a few times, then hard gold is a must. If it is a prototype that will be discarded after a few insertions, some vendors like JLCPCB offer ENIG board plating that will drastically reduce the price. Of course, use cheap sockets on discardable prototype boards.

A few early PCBs had smaller edge connectors on the top edge of the board. The IMSAI PIO and SIO are examples. Later boards were designed using shrouded or unshrouded header pins to increase reliability and reduce cost, not to mention eliminating the extra routing around the connectors.

One other important aspect of the edge connector is the chamfer or bevel. The IEEE 696 specifies a 0.015" × 45° chamfer which reduces the insertion force. The angle is less important than the distance. This will be discussed in greater detail in Suggestions for New Designs.

Suggestions for New Designs

Board Outline

Consult the IEEE 696 specifications for the board outline dimensions. The specification says to “break all sharp corners.” Our outlines include a 0.015" 45° “break” on all 4 90° corners. This will not increase the cost of the PCB. The two inside corners for the card edge connector should be routed with as small a radius as supported by the vendor without increasing the cost.

Signal Routing

In general, your layout should prioritize in this order:

Ground plane
Power planes
Critical signal routing
General signal routing

This is more difficult that it may seem at first on a 2-layer PCB. This is why legacy PCBs were generally 2 oz copper because there was not enough routing space for a proper ground plane. If your PCB is dense or has high-speed logic circuits, you may want to consider a 4-layer PCB and use the inner layers for the ground and power planes. Believe it or not, there were some legacy 4-layer PCBs produced. The cost delta for 4-layer is very dependent on the vendor, so going that route may limit your vendor choices.

Card Edge Connector

The edge connector PCB layout suggested by most fabs today is somewhat different from the legacy layouts that we used to do. We used to lay out the fingers well past the bottom edge of the board outline, and connect them together with short traces to a heavy bus trace. It looked a bit like a comb. The fabs would then use the bus trace to attach the cathode before immersion into the plating solutions. Once the fingers were plated, the PCB was routed and chamfered, cutting right through the extra plated metal.

Today, the fabs have the ability to easily manipulate the gerbers and attach whatever sorts of electrical plating features they need. They want the gerbers to represent the finished PCB. They also don’t want to have to bevel through the fingers. Some fabs will reject a job if the fingers extend past the beveling area. There is a good reference article from JLCPCB (who ironically doesn’t offer gold fingers) here.

Our S-100 fingers are oval shaped. While this shape was rather difficult to do back in the manual tape-up days, it is very easy to accomplish with today’s CAD tools. This shape is ideal from several standpoints.

The lack of sharp corners reduce reflections in the same way as avoiding 90° sharp turns in the trace routing (although this is not a serious issue at S-100 bus speeds).
The rounded insertion profile is less likely to delaminate due to mechanical stress.
The rounded edge near the board edge makes a much neater transition to the plating bar traces.
It uses slightly less Nickel and Gold plating due to the reduced surface area.

The outside edges of the fingers are 0.015" (0.38mm) from the routed board edge, which is the IEEE 696 chamfer specification. Beveling tolerances may remove a tiny bit of the finger tip, or leave a tiny bit of the plating bar connection. Pins 1, 50, 51, and 100 are 50% wider for power and ground.

Asian Vendors

This section lists some of the larger Asian vendors, sorted by ascending estimated price for 10 PCBs. Quantities of less than 10 for most vendors do not meaningfully reduce the total price. Usually the minimum sweet spot is around 25 PCBs.

PCBnet (imagineering, China fab): $12.71 ea. PCBnet also offers Taiwan and USA fabs. The USA fab would not quote a price without a phone call, which is why they are not included in the North American Vendors list.
Bittele: $24.49 ea. Bittele is a Canadian company who typically outsources fabs like this to China.
PCBnet (imagineering, Taiwan fab): $33.60 ea. PCBnet also offers China and USA fabs. The USA fab would not quote a price without a phone call, which is why they are not included in the North American Vendors list.
PCBWay: $36.14 ea. PCBWay offers 10µ" Gold over 120µ" Nickel fingers.

JLCPCB: $4.96 ea. JLCPCB does not offer hard gold fingers. They do offer ENIG, which is only recommended for limited use prototypes. You will have to specify the entire PCB as ENIG. The other problem with JLCPCB is that their finger chamfer is 30°, not 45°, which means that some of the fingers near the edge will be routed away. Probably not enough to worry about, but it is not IEEE 696 compliant.

North American Vendors

This section lists some of the larger North American vendors, sorted by ascending estimated price for 10 PCBs. Quantities of less than 10 for most vendors do not meaningfully reduce the total price. Usually the minimum sweet spot is around 25 PCBs.

PCBCart: $63.50 ea @ 4-weeks. PCBCart (a.k.a. Advanced Circuits) does not specify the thicknesses of their gold fingers, but otherwise offers IEEE 696 compliance.
Sunstone: $84.97 ea. Use the Sunstone PCBpro Full Feature service to get hard gold edge connectors. This is the only listed vendor we have not used so far.