Burn-in and Test Board Design Guidelines.

Following is information on typical design guidelines for burn-in and test boards.

Each PC board design must be treated uniquely, as each layout is a combination of proper material selections, mechanical constraints and electrical wizardry. Hence, the following is a list of design goals that should be reviewed for each layout.

Maximum Voltage Spacing

Generally, line spaces of a minimum 0.010" can be maintained for differentials of less than 50V. However, Loranger has built boards with up to 1,000-vol differentials with consequent increased trace spacing.

Minimum Widths and Spaces

Line widths and spaces of 0.010" can be easily obtained, however, 0.025" minimum spaces and traces are preferred to accommodate manufacturing cost and wear concerns. Smaller dimensions add significant cost but Loranger has produced 0.004" traces and spaces with success.

Multi layer Design

Where possible, two-sided PC boards with plated-thru holes are preferred for economy, reliability and ease-of-repair; however, multi layers of 4 thru 26 layers are available.

Plated-Thru-Holes
Circuit Layout

Loranger normally prefers a 0.5" "circuit-free" zone around the entire perimeter of the PC board, save the edge finger area, for manufacturability, sharpness of the layout and mechanical attachment spacing for the framing, such as the Isolation / Support Frame.

Economic concerns generally force our designers to consider four rules for PTH sizing:

1.
Minimize the number of different hole sizes to avoid drill bit changes.
2.
Ideally design the PTHs to be 0.010" over the diagonal of the socket or device lead fitting the hole for good solderability and ease of component loading.
3.
Refrain from holes under 0.031" diameter for manufacturability concerns (drill bits below 0.031" break and wander easily, adding significant cost for holes below 0.015" diameter.)
4.
Respect the ratio of hole diameter to PC board thickness (1 :3 maximum). To assure accuracy and economy, 0.040" minimum holes to 0.062" thick PC boards work well.
Decoupling

Bulk Decoupling - for low frequency decoupling, use 46 to 200 microfarad capacitors between the supply voltage and common return. These capacitors should be solid tantalum capacitors which have better transient response than most other large value capacitors and put more capacitance into a small package, which simplifies board layout. Half the bulk capacitance should be placed near the point where the supplies enter the board and the other half at the far side of the DUT so the array lies between the bulk decoupling capacitors. It is important that both the capacitance and voltage ratings of solid tantalum capacitors be derated for use at 125°C, the maximum operating temperature specified by most capacitor manufacturers. Although solid tantalum capacitors are expensive, their cost is insignificant when compared to the value of the DUT they protect.

Local Decoupling - to avoid high-frequencies overcomes the voltage spikes which develop on the power supply pins of the memory devices when they are turned on or off by dynamic control signals. Layout the board for a 0.1 microfarad capacitor per DUT. These local decoupling capacitors should be located as close as physically possible to each chip and have the shortest possible traces from their respective power supply to ground. To further reduce inductance, these traces should be as wide as room will permit.

Pad Areas Around PTHs

For soundness of solder joints, design with a minimum of 50% more pad area than the PTH area (0.010" minimum annular ring).

Power Distribution

The single most important aspect of successful memory burn-in board design is good power distribution accompanied with carefully designed decoupling and power gridding.

Other Design Issues

The following are guidelines for the proper techniques to be used when designing burn-in boards for RAMS, ROMs, microprocessors and other electrically-delicate devices. Based upon individual customer requirements, Loranger incorporates these and other (proprietary) techniques to produce high-quality burn-in boards with exceptional signal integrity. Separate cost must be quoted, however, to uniquely tune a design to a specific device type.

Grids

Grids for power distribution are important layout considerations. Both voltage and ground buses should be bused up and down the burn-in board throughout every device position. Providing multiple paths through the array reduces the effective inductance of the power distribution system. Traces running the power supply voltages throughout the array should be as wide as possible.

Speed and Signal Distribution

The second most important aspect of successful burn-in board design.

Blind, Buried and Built-up Via Holes

Minimize the use of vias in multi layer designs for circuit integrity and cost.

Feed-Thru Holes

Minimize the number of feed-thru holes that do not have a component lead soldered into them to enhance reliability.

Solder Mask & Conformal Coating

Solder masks are generally required, however where insulation needs are great or where humidity or other atmospheric contaminates are present, conformal coating may be recommended. Consult Loranger for recommendation of special applications.

Wire Size Selection

In general, the wire sizes selected should be one size bigger than what is required to insure safety, minimize voltage drops and crosstalk between lines. Wire insulation is normally silicone or Teflon plastic for all oven atmospheres up to and including 200°C. Above 200°C, a silicone glass braid is normally chosen for insulation. Finally, standard wire is generally preferred over solid conductors. Outside the oven environment, THHN insulation is normally used

 
Uninsulated Wire Data
AWG
Diameter (in)
Circular (mils)
Current-Carrying Capacity
Lbs per 1000 ft
DC Resistance
(m½ / ft)
24
0.02
404
1
1.2
51.0
22
0.025
640
1.5
1.9
32.0
20
0.032
1,020
2.4
3.1
20.0
18
0.04
1,620
3.8
4.9
13.0
16
0.051
2,580
6
7.8
8.0
14
0.064
4,110
9.7
12
5.0
12
0.081
6,530
15
20
3.0
10
0.102
10,380
24
31
2.0
8
0.129
16,510
39
50
1.3
6
0.162
26,204
62
79
0.8
4
0.204
41,740
99
126
0.5
2
0.258
66,400
157
201
0.32
0
0.325
106,000
252
320
0.2
00
0.365
133,000
316
403
0.16
000
0.410
167,800
386
508
0.13
NOTE :
Current-carrying capacity is based on maximum current that will cause less than 10°C rise above ambient.
Recommended Conductor Spacing
Applied Voltage
Sea Level (in)
6,500 ft (in)
10,000 ft (in)
0 to 200V
0.010
0.013
0.018
300V
0.026
0.033
0.038

400V

0.044
0.057
0.065
500V
0.069
0.087
0.100
600V
0.096
0.122
0.140
700V
0.131
0.165
0.190
800V
0.166
0.209
0.240
900V
0.207
0.261
0.300
1000V
0.252
0.318
0.365
NOTE 1:
Voltage and altitude affect minimum spacing requirements.
NOTE 2:
Humidity and salt contamination environments require further derating on the above values. Conformal coating enhances reliability by protecting the board from contamination and handling abuse.
PC Board Solder Selection
Solder Type Plastic / Eutectic Point Recommended Temp Use (max) Composition
Type I
183°C
143°C
60% Sn 40% Pb
or Equiv
Type II
221°C
175°C
96% Sn 4% Ag
or Equiv
Type III A
310°C
250°C
97.5% Pb 1% Sn 1.5% Ag or Equiv
Type III B
308°C
250°C
5% Sn 95% Pb
or Equiv
Summary

The foregoing rules are general, sound engineering design practices. Unique needs may force the modification or enhancement of these rules. Other concerns for electrical noise, line termination and decoupling may further augment the design and layouts. PTH pads, conductor paths, edge finger plating, nomenclature, solder mask and fabrication details are covered by Loranger's Manufacturing Process Specifications (MPS #3400 Series).

About Us


Loranger International Corporation designs and manufactures burn-in and test sockets, printed circuit boards and environmental chamber systems for the electronics manufacturing industry.

Loranger International Corporation sockets are optimized for today’s small, fine pitch electronic packages including QFN, as well as for older style packages.

Loranger International Corporation burn-in and test sockets are used worldwide in programs including defense, guidance, automotive, music, communication and space exploration.

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Loranger International, Inc.
817 Fourth Ave.
Warren, PA 16365
303 Brokaw Road
Santa Clara, CA 95050
Phone
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