Staking is the process of bonding and securing components or parts to PWA's and electronic assemblies by means of an adhesive material. The main purpose is to protect and support parts that may be damaged by vibration, shock, or handling. Reliability: Reliable staking results from proper design, control of equipment, materials, work environments, and careful workmanship by trained and certified personnel.
Design Considerations for Staking
The staking material shall be a noncorrosive, electrically insulative material, with dielectric properties (permittivity and dissipation factor) that will not change or adversely affect the performance characteristics of the parts being staked or their associated circuitry.
The staking material selected shall provide adequate mechanical support to allow the item to survive vibration levels imposed during end-item use. Rigid staking material with a low thermal expansion coefficient is generally desirable. For special cases where parts, sensitive to thermal/mechanical stress are used, application of resilient materials may be required.
The staking material must be compatible with, and adhere to, the printed wiring assembly (PWA) or substrate, the part staked, and the conformal coating to be applied.
The staking material shall exhibit hydrolytic (a chemical reaction during which molecules of water (H
2O) are split into hydrogen cations (H +) (conventionally referred to as protons) and hydroxide anions (OH -) in the process of a chemical mechanism 4) and thermal stability under high-vacuum and thermal-vacuum conditions. The material shall meet program and contractual outgassing, offgassing, and flammability requirement. The staking compound selected must not negate the stress relief in parts leads or on jumper wires.
Staking material selection shall take into consideration the system operating temperatures and the material glass transition temperatures as specified on the approved engineering documentation in order to minimize stress on parts and jumper wiring during operational thermal cycling.
Staking material must not be applied to areas where induced stress will cause damage, such as under a dual-in-line (DIP) integrated circuit or flat pack device.
The staking material selected shall be curable under temperature conditions compatible with the PWA on which it is located.
The manufacturer’s specification for the parts-by-weight (PBW) mixing of the two-part resin system shall be followed. If the required weight of filler added to a formulation is not listed in a manufacturer’s specification; it is determined by the materials engineer. Only the weight in grams of the filler is entered on the Mixing Record. For example, Table 3 is the formulation for GSFC approved staking-grade resin mix that would appear on the mixing record. Approved fluorescent dyes may be added into the staking formulation. Resin formulation is as follows:
2 Thickener 2.75
*Must be kept at 125°C for at least forty-eight (48) hours before use.
After the formulation has been mixed, it should be degassed or deaerated until all entrapped air is removed.
Inspection Methods for Staking
Workmanship and Adhesion Requirements: Satisfaction of requirements shall be verified by visual inspection using 4X to 10X power magnification. Higher magnification may be used, as necessary, to inspect suspected anomalies or defects.
Tackiness and Adhesion: Gentle pressure shall be used to inspect for tackiness and adhesion. For this purpose, lint free gloves or finger cots shall be worn.
Hardness: When applicable, hardness shall be measured in accordance with ASTM-D-2240, "Standard Method of Test for Rubber Property Durometer Hardness."
The staking material shall adhere to the intended surfaces as per paragraph 9.2-1 of NASA-STD-8739-1.
The staking material shall be free from contamination as per paragraph 9.2-3b of NASA-STD-8739-1.
Jumper wires shall be staked a minimum of every 2.54 cm (1 inch) and every change of direction, outside of the radius of curvature as per paragraph 9.2-4 of NASA-STD-8739-1.
All axial lead solid slug tantalum capacitors shall be staked as per paragraph 9.2-4 of NASA-STD-8739-1.
The staking material shall be tack-free when cured as per paragraph 9.2-5 of NASA-STD-8739-1.
Staking material meets the hardness requirements as per paragraph 9.2-6 of NASA-STD-8739-1.
Staking Rejection Criteria
Staking material used after shelf life expiration as per paragraph 6.7-lb of NASA-STD-8739-1.
Staking material bridges between the PWB and the bottom of the DIP’s or flatpacks as per paragraph 9.2-1 of NASA-STD-8739-1.
Staking material fills the stress relief areas as per paragraph 9.2-3a of NASA-STD-8739-1.
Staking material encloses the part lead as per paragraph 9.2-3a of NASA-STD-8739-1.
Rigid staking has been applied directly to glass bodied parts as per paragraph 9.2-3c of NASA-STD-8739-1.
Array of Vertically Mounted Rectangular Parts
Single Vertically Mounted Rectangular Part